Magnetic recording medium and magnetic recording and reproducing device

ABSTRACT

The magnetic recording medium includes a non-magnetic support, a magnetic layer including a ferromagnetic powder on one surface of the non-magnetic support, and a back coating layer including a non-magnetic powder on the other surface of the non-magnetic support, in which a difference (S after −S before ) between a spacing S after  measured by optical interferometry regarding a surface of the back coating layer after methyl ethyl ketone cleaning and a spacing S before  measured by optical interferometry regarding the surface of the back coating layer before methyl ethyl ketone cleaning is greater than 0 nm and equal to or smaller than 30.0 nm, and the non-magnetic support is an aromatic polyamide support having a moisture absorption of 2.2% or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C 119 to Japanese PatentApplication No. 2019-086682 filed on Apr. 26, 2019. The aboveapplication is hereby expressly incorporated by reference, in itsentirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic recording medium and amagnetic recording and reproducing device.

2. Description of the Related Art

A magnetic recording medium (for example, see JP-H9-227883A) is arecording medium useful as a data storage medium for storing a largeamount of data (information) for a long period of time.

SUMMARY OF THE INVENTION

The recording of data on a magnetic recording medium and the reproducingthereof are generally performed by mounting the magnetic recordingmedium on a magnetic recording and reproducing device (referred to as adrive) and allowing the magnetic recording medium to run in the drive.In order to prevent occurrence of errors during the recording andreproducing, it is desired to stabilize the running (improve runningstability) of the magnetic recording medium in the drive.

The magnetic recording medium used for data storage is used in a datacenter in which a temperature and humidity are managed. Meanwhile, inthe data center, power saving is necessary for reducing the cost. Forrealizing the power saving, the managing conditions of the temperatureand humidity of the data center can be alleviated compared to thecurrent state, or the managing may not be necessary. However, in a casewhere the managing conditions of the temperature and humidity arealleviated or the managing is not performed, the magnetic recordingmedium is assumed to be exposed to an environmental change caused by theweather change or the seasonal change or to be held in varioustemperature and humidity environments. An example of an environmentalchange is a temperature change from a low temperature to a hightemperature under high humidity. In addition, an example of thetemperature and humidity environments is a high temperature and highhumidity environment. Therefore, it is desirable that the running of themagnetic recording medium in the drive can be stabilized even after suchan environmental change and after storage in such a temperature andhumidity environment.

The magnetic recording medium generally has a configuration of includinga non-magnetic support and a magnetic layer containing a ferromagneticpowder. In addition, as disclosed in claim 2 of JP1997-227883A(JP-H9-227883A), a back coating layer is formed on a surface of thenon-magnetic support of the magnetic recording medium opposite to asurface provided with the magnetic layer. In regard to the non-magneticsupport, for example, paragraph 0062 of JP1997-227883A (JP-H9-227883A)discloses various films that can be used as the non-magnetic support.

The inventors have conducted studies regarding the above points, and itwas clear that running stability of a magnetic recording mediumincluding an aromatic polyamide support as a non-magnetic support isdeteriorated, in a case where the medium recording medium is stored in ahigh temperature and high humidity environment, after the occurrence ofa temperature change from a low temperature to a high temperature underhigh humidity.

According to one aspect of the invention, an object thereof is toprevent a deterioration in running stability of a magnetic recordingmedium including an aromatic polyamide support after a temperaturechange from a low temperature to a high temperature under high humidityand after being held in a high temperature and high humidityenvironment.

According to one aspect of the invention, there is provided a magneticrecording medium including:

a non-magnetic support;

a magnetic layer including a ferromagnetic powder on one surface of thenon-magnetic support; and

a back coating layer including a non-magnetic powder on the othersurface of the non-magnetic support,

in which a difference (S_(after)−S_(before)) between a spacing S_(after)measured by optical interferometry regarding a surface of the backcoating layer after methyl ethyl ketone cleaning and a spacingS_(before) measured by optical interferometry regarding the surface ofthe back coating layer before methyl ethyl ketone cleaning (hereinafter,also referred to as a “spacing difference (S_(after)−S_(before)) beforeand after methyl ethyl ketone cleaning” or simply a “difference(S_(after)−S_(before))”) is greater than 0 nm and equal to or smallerthan 30.0 nm, and the non-magnetic support is an aromatic polyamidesupport having a moisture absorption of 2.2% or less.

In an embodiment, the difference (S_(after)−S_(before)) may be 4.0 nm to28.0 nm.

In an embodiment, the aromatic polyamide support may have a moistureabsorption of 2.0% or less.

In an embodiment, the aromatic polyamide support may have a moistureabsorption of 1.0% or more and 2.0% or less.

In an embodiment, the magnetic recording medium may further include anon-magnetic layer including a non-magnetic powder between thenon-magnetic support and the magnetic layer.

In an embodiment, the magnetic recording medium may be a magnetic tape.

One aspect of the invention relates to a magnetic recording andreproducing device including the magnetic recording medium and amagnetic head.

According to one embodiment of the invention, it is possible to providea magnetic recording medium including an aromatic polyamide support, inwhich a deterioration in running stability is slight, even in a case ofbeing stored in a high temperature and high humidity environment after atemperature change from a low temperature to a high temperature underhigh humidity, and a magnetic recording and reproducing device includingthe magnetic recording medium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Magnetic Recording Medium

One aspect of the invention relates to a magnetic recording mediumincluding a non-magnetic support, a magnetic layer including aferromagnetic powder on one surface of the non-magnetic support, and aback coating layer including a non-magnetic powder on the other surfaceof the non-magnetic support, in which a difference(S_(after)−S_(before)) between a spacing S_(after) measured by opticalinterferometry regarding a surface of the back coating layer aftermethyl ethyl ketone cleaning and a spacing S_(before) measured byoptical interferometry regarding the surface of the back coating layerbefore methyl ethyl ketone cleaning is greater than 0 nm and equal to orsmaller than 30.0 nm, and the non-magnetic support is an aromaticpolyamide support having a moisture absorption of 2.2% or less.

In the invention and the specification, the “methyl ethyl ketonecleaning” means ultrasonic cleaning (ultrasonic output: 40 kHz)performed for 100 seconds by dipping a test piece cut out from themagnetic recording medium into methyl ethyl ketone (200 g) at a liquidtemperature of 20° C. to 25° C. In a case where the magnetic recordingmedium to be cleaned is a magnetic tape, a test piece having a length of5 cm is cut out and subjected to methyl ethyl ketone cleaning. A widthof the magnetic tape and a width of the test piece cut out from themagnetic tape is normally ½ inches. 1 inch is 0.0254 meters. Regarding amagnetic tape having a width other than the width of ½ inches, a testpiece having a length of 5 cm may be cut out and subjected to methylethyl ketone cleaning. In a case where the magnetic recording medium tobe cleaned is a magnetic disk, a test piece having a size of 5 cm×1.27cm is cut out and subjected to methyl ethyl ketone cleaning. Themeasurement of the spacing after the methyl ethyl ketone cleaningdescribed below is performed, after the test piece after the methylethyl ketone cleaning is left in an environment of a temperature of 23°C. and relative humidity of 50% for 24 hours.

In the invention and the specification, the “surface of the back coatinglayer” of the magnetic recording medium is identical to the surface ofthe magnetic recording medium on the back coating layer side.

In the invention and the specification, the spacing measured by opticalinterferometry regarding the surface of the back coating layer of themagnetic recording medium is a value measured by the following method.

In a state where the magnetic recording medium (specifically, the testpiece. The same applies hereinafter) and a transparent plate-shapedmember (for example, glass plate or the like) are overlapped onto eachother so that the surface of the back coating layer of the magneticrecording medium faces the transparent plate-shaped member, a pressingmember is pressed against the side of the magnetic recording mediumopposite to the back coating layer side at pressure of 0.5 atm (1atm=101325 Pa (pascals). In this state, the surface of the back coatinglayer of the magnetic recording medium is irradiated with light throughthe transparent plate-shaped member (irradiation region: 150,000 to200,000 μm²), and a spacing (distance) between the surface of the backcoating layer of the magnetic recording medium and the surface of thetransparent plate-shaped member on the magnetic recording medium isacquired based on intensity (for example, contrast of interferencefringe image) of interference light generated due to a difference in alight path between reflected light from the surface of the back coatinglayer of the magnetic recording medium and reflected light from thesurface of the transparent plate-shaped member on the magnetic recordingmedium. The light emitted here is not particularly limited. In a casewhere the emitted light is light having an emission wavelength over acomparatively wide wavelength range as white light including lighthaving a plurality of wavelengths, a member having a function ofselectively cutting light having a specific wavelength or a wavelengthother than wavelengths in a specific wavelength range, such as aninterference filter, is disposed between the transparent plate-shapedmember and a light receiving unit which receives reflected light, andlight at some wavelengths or in some wavelength ranges of the reflectedlight is selectively incident to the light receiving unit. In a casewhere the light emitted is light (so-called monochromatic light) havinga single luminescence peak, the member described above may not be used.The wavelength of light incident to the light receiving unit can be setto be 500 to 700 nm, for example. However, the wavelength of lightincident to the light receiving unit is not limited to be in the rangedescribed above. In addition, the transparent plate-shaped member may bea member having transparency through which emitted light passes, to theextent that the magnetic recording medium is irradiated with lightthrough this member and interference light is obtained.

The interference fringe image obtained by the measurement of the spacingdescribed above is divided into 300,000 points, a spacing of each point(distance between the surface of the back coating layer of the magneticrecording medium and the surface of the transparent plate-shaped memberon the magnetic recording medium side) is acquired, this spacing isshown with a histogram, and a mode of this histogram is set as thespacing. The difference (S_(after)−S_(before)) is a value obtained bysubtracting a mode before the methyl ethyl ketone cleaning from a modeafter the methyl ethyl ketone cleaning of the 300,000 points.

Two test pieces from the same magnetic recording medium are cut out, avalue S_(before) of the spacing is obtained without performing themethyl ethyl ketone cleaning with respect to the one test piece, and avalue S_(after) of the spacing is obtained after performing the methylethyl ketone cleaning with respect to the other test piece, and thedifference (S_(after)−S_(before)) may be obtained. Alternatively, thedifference (S_(after)−S_(before)) may be obtained by acquiring values ofthe spacing after performing the methyl ethyl ketone cleaning withrespect to the test piece, with which the value of the spacing beforethe methyl ethyl ketone cleaning is acquired.

The above measurement can be performed by using a commercially availabletape spacing analyzer (TSA) such as Tape Spacing Analyzer manufacturedby Micro Physics, Inc., for example. The spacing measurement of theexamples was performed by using Tape Spacing Analyzer manufactured byMicro Physics, Inc.

In the invention and the specification, the “aromatic polyamide” means apolyamide containing an aromatic skeleton, and the “aromatic polyamidesupport” means a support containing an aromatic polyamide film. Thearomatic polyamide support can be a single-layer aromatic polyamidefilm, or a laminated film of two or more aromatic polyamide films havingthe same components or two or more aromatic polyamide films havingdifferent components. The “aromatic polyamide film” refers to a film inwhich the component that occupies the largest amount among thecomponents constituting the film based on mass is an aromatic polyamide.The laminated film may optionally include an adhesive layer or the likebetween two adjacent layers.

In the invention and the specification, the moisture absorption of thearomatic polyamide support is a value determined by the followingmethod.

A test piece (for example, a test piece having a mass of several grams)cut out from the aromatic polyamide support, moisture absorption ofwhich is to be measured, is dried in a vacuum dryer at a temperature of180° C. and under a pressure of 100 Pa or less until a constant weightis obtained. The mass of the dried test piece is defined as W1. W1 is avalue measured in a measurement environment of a temperature of 23° C.and relative humidity of 50% within 30 seconds after being taken out ofthe vacuum dryer. Next, the mass of this test piece after being placedin an environment of a temperature of 25° C. and relative humidity of75% for 48 hours is defined as W2. W2 is a value measured in ameasurement environment of a temperature of 23° C. and relative humidityof 50% within 30 seconds after being taken out of the environment. Themoisture absorption is calculated by the following equation.Moisture absorption(%)=[(W2−W1)/W1]×100

For example, it is also possible to obtain the moisture absorption ofthe aromatic polyamide support by the method described above, afterremoving portions other than the aromatic polyamide support such as themagnetic layer from the magnetic recording medium by a well-known method(for example, film removal using an organic solvent).

Hereinafter, a surmise of the inventors will be described in regard to apoint that it is possible to prevent a deterioration in runningstability after a temperature change from a low temperature to a hightemperature under high humidity and being stored in a high temperatureand high humidity environment (hereinafter, also simply referred to as a“deterioration in running stability”) by the magnetic recording medium.

The recording of data on a magnetic recording medium and the reproducingthereof are generally performed by allowing the magnetic recordingmedium to run in the drive. In general, the surface of the back coatinglayer comes into contact with a drive constituent element in the drive.As an example of the drive constituent element, a roller performingsending and/or winding of the tape-shaped magnetic recording medium(magnetic tape) is used. Here, it is thought that, in a case where acontact state between the drive constituent element and the surface ofthe back coating layer is unstable, running stability of the magneticrecording medium in the drive is deteriorated.

Meanwhile, it is thought that, in a case where temperature change from alow temperature to a high temperature occurs under high humidity,condensation (attachment of moisture) occurs on the surface of the backcoating layer of the magnetic recording medium. It is surmised that thepresence of moisture causing an increase in a coefficient of frictionduring the contact between the surface of the back coating layer and thedrive constituent element is a reason of a deterioration in runningstability. Therefore, it is thought that, a decrease in amount ofmoisture attached to the surface of the back coating layer, in a casewhere a temperature change from a low temperature to a high temperatureoccurs under high humidity, allows the prevention of an increase incoefficient of friction, thereby preventing a deterioration in runningstability.

However, a portion (projection) which mainly comes into contact(so-called real contact) with the drive constituent element during thecontact between the surface of the back coating layer and the driveconstituent element, and a portion (hereinafter, referred to as a “baseportion”) having a height lower than that of the portion described aboveare normally present on the surface of the back coating layer. Theinventors have thought that the spacing described above is a value whichis an index for a distance between the drive constituent element and thebase portion during the contact between the surface of the back coatinglayer and the drive constituent element. However, it is surmised that,in a case where some components are present on the surface of the backcoating layer, as the amount of the components interposed between thebase portion and the drive constituent element increases, the spacing isnarrowed. Meanwhile, in a case where the components are removed by themethyl ethyl ketone cleaning, the spacing spreads, and accordingly, thevalue of the spacing S_(after) after the methyl ethyl ketone cleaning isgreater than the value of the spacing S_(before) before the methyl ethylketone cleaning. Accordingly, it is thought that the difference(S_(after)−S_(before)) of the spacings before and after the methyl ethylketone cleaning can be an index for the amount of the componentinterposed between the base portion and the drive constituent element.

In regards to this point, the inventors have thought that the presenceof the component removed by the methyl ethyl ketone cleaning on thesurface of the back coating layer promotes the attachment of moisture tothe surface of the back coating layer, in a case where a temperaturechange from a low temperature to a high temperature occurs under highhumidity. Accordingly, the inventors have thought that, in a case wherethe difference (S_(after)−S_(before)) of the spacings before and afterthe methyl ethyl ketone cleaning is decreased, that is, a decrease inthe amount of the component contributes to prevention of the attachmentof moisture, and as a result, this allows the prevention of an increasein coefficient of friction. The inventors have thought that thiscontributes to preventing a deterioration in running stability due to atemperature change from a low temperature to a high temperature underhigh humidity. With respect to this, according to the studies of theinventors, a correlation is not found between a value of the differenceof spacings before and after cleaning using a solvent other than methylethyl ketone, for example, n-hexane, and a value of the difference ofspacings before and after methyl ethyl ketone cleaning. It is surmisedthat this is because the component cannot be removed or cannot besufficiently removed in the n-hexane cleaning.

Details of the component are not clear. Merely as a surmise, theinventors thought that the component may be a component, a molecularweight of which is greater than that of an organic compound normallyadded to the back coating layer as an additive. The inventors havesurmised as follows regarding one aspect of this component. In anembodiment, the back coating layer is formed by applying a back coatinglayer forming composition including a binding agent and a curing agentonto a non-magnetic support, in addition to the non-magnetic powder, andperforming a curing treatment. With the curing treatment here, it ispossible to allow a curing reaction (crosslinking reaction) between thebinding agent and the curing agent. However, it is thought that thebinding agent which is not subjected to the curing reaction with thecuring agent or the binding agent which is insufficiently subjected tothe curing reaction with the curing agent is easily separated from theback coating layer and may be present on the surface of the back coatinglayer. The inventors have surmised that the moisture easily adsorbed tothe binding agent (for example, functional group including this bindingagent) is a reason of the promotion of the attachment of moisture to thesurface of the back coating layer, in a case where a temperature changefrom a low temperature to a high temperature occurs under high humidity.

In addition, it is thought that, in a case where the non-magneticsupport absorbs a large amount of water during storage in a hightemperature and high humidity environment, a coefficient of frictionduring the contact between the surface of the back coating layer of themagnetic recording medium including the non-magnetic support and thedrive constituent element may increase. This is considered as one ofreasons for a deterioration in running stability after a temperaturechange from a low temperature to a high temperature under high humidityand storage in a high temperature and high humidity environment. Sincethe aromatic polyamide support generally tends to absorb moisture, it issurmised that such an increase in the coefficient of friction easilyoccurs. In contrast, it is thought that, in the magnetic recordingmedium, the aromatic polyamide support has a moisture absorption of 2.2%or less, and accordingly, the amount of water absorbed by the aromaticpolyamide support during storage in a high temperature and high humidityenvironment is small. It is surmised that this also contributes topreventing a deterioration in running stability.

However, the above description is merely a surmise of the inventors andthe invention is not limited thereto. Hereinafter, the magneticrecording medium will be further described in detail.

Non-Magnetic Support

The magnetic recording medium includes an aromatic polyamide supporthaving a moisture absorption of 2.2% or less as a non-magnetic support.From a viewpoint of preventing a deterioration in running stability, themoisture absorption of the aromatic polyamide support is 2.2% or less,preferably 2.1% or less, and more preferably 2.0% or less, even morepreferably 1.9% or less, still preferably 1.8% or less, still morepreferably 1.7% or less, still even more preferably 1.6% or less, andstill furthermore preferably 1.5% or less. In addition, the moistureabsorption of the aromatic polyamide support is, for example, 0% ormore, more than 0%, 0.1% or more, 0.3% or more, 0.5% or more, 0.7% ormore, 1.0% or more, or 1.2% or more. From a viewpoint of preventing adeterioration in running stability, since the aromatic polyamide supportpreferably has a low moisture absorption, the moisture absorption canalso be 0%. It is also preferable to use the aromatic polyamide supporthaving a low moisture absorption as the non-magnetic support of themagnetic recording medium, from a viewpoint of preventing deformation ofthe magnetic recording medium after long-term storage. For example, itis preferable that the tape-shaped magnetic recording medium (magnetictape) includes the aromatic polyamide support having a low moistureabsorption, from a viewpoint of preventing deformation of the magnetictape in the tape width direction after long-term storage. In addition,in the magnetic tape, the aromatic polyamide support preferably has aYoung's modulus of 3000 N/mm² or more in a longitudinal direction and4000 N/mm² or more in a width direction. From a viewpoint of increasingthe capacity of the magnetic recording medium, a surface roughness ofone or both surfaces of the aromatic polyamide support is preferably 10nm or less as a center line average roughness Ra.

An aromatic ring contained in the aromatic skeleton of the aromaticpolyamide is not particularly limited, and specific examples of thearomatic ring include a benzene ring and a naphthalene ring. Themoisture absorption of the aromatic polyamide support can be controlledby a type and a ratio of the constituent units constituting the aromaticpolyamide. For details of the aromatic polyamide support that can beused as the non-magnetic support of the magnetic recording medium,well-known technologies can be referred to, for example, paragraphs 0007to 0054 and Examples of JP1997-176306A (JP-H9-176306A) can be referredto. The non-magnetic support may be a biaxially stretched film, and maybe a film subjected to corona discharge, plasma treatment, easy adhesiontreatment, heat treatment, and the like.

Spacing Difference (S_(after)−S_(before)) Before and after Methyl EthylKetone Cleaning

The spacing difference (S_(after)−S_(before)) before and after methylethyl ketone cleaning measured by optical interferometry regarding thesurface of the back coating layer of the magnetic recording medium isgreater than 0 nm and equal to or smaller than 30.0 nm. A difference(S_(after)−S_(before)) of 30.0 nm or less can contribute to preventing adeterioration in running stability in the magnetic recording medium.From this viewpoint, the difference (S_(after)−S_(before)) is equal toor smaller than 30.0 nm, preferably equal to or smaller than 29.0 nm,more preferably equal to or smaller than 28.0 nm, even more preferablyequal to or smaller than 27.0 nm, still preferably equal to or smallerthan 26.0 nm, and still more preferably equal to or smaller than 25.0nm. As will be described later in detail, the difference(S_(after)−S_(before)) can be controlled by a surface treatment of theback coating layer in a manufacturing step of the magnetic recordingmedium.

In addition, the inventors thought that, as the spacing difference(S_(after)−S_(before)) before and after the methyl ethyl ketone cleaningbecomes 0 nm, in a case where the surface treatment of the back coatinglayer is performed, a large amount of the component (for example,lubricant) contributing to the improvement of running stability isremoved from the magnetic recording medium. However, this is merely asurmise of the inventors and the invention is not limited thereto. Fromthis viewpoint, the spacing difference (S_(after)−S_(before)) of themagnetic recording medium is greater than 0 nm, preferably equal to orgreater than 1.0 nm, more preferably equal to or greater than 2.0 nm,even more preferably equal to or greater than 3.0 nm, still preferablyequal to or greater than 4.0 nm.

Next, the magnetic layer, the back coating layer, and the non-magneticsupport of the magnetic recording medium, and the non-magnetic layer,which is randomly included will be further described.

Magnetic Layer

Ferromagnetic Powder

As the ferromagnetic powder included in the magnetic layer, a well-knownferromagnetic powder used as one kind or in combination of two or morekinds can be used as the ferromagnetic powder used in the magnetic layerof various magnetic recording media. It is preferable to useferromagnetic powder having a small average particle size, from aviewpoint of improvement of recording density. From this viewpoint, anaverage particle size of the ferromagnetic powder is preferably 50 nm orless, more preferably 45 nm or less, even more preferably 40 nm or less,further more preferably 35 nm or less, and still preferably 30 nm orless, still more preferably 25 nm or less, and still even morepreferably 20 nm or less. On the other hand, from a viewpoint ofmagnetization stability, the average particle size of the ferromagneticpowder is preferably 5 nm or more, more preferably 8 nm or more, evenmore preferably 10 nm or more, still preferably 15 nm or more, and stillmore preferably 20 nm.

Hexagonal Ferrite Powder

As a preferred specific example of the ferromagnetic powder, hexagonalferrite powder can be used. For details of the hexagonal ferrite powder,descriptions disclosed in paragraphs 0012 to 0030 of JP2011-225417A,paragraphs 0134 to 0136 of JP2011-216149A, paragraphs 0013 to 0030 ofJP2012-204726A, and paragraphs 0029 to 0084 of JP2015-127985A can bereferred to, for example.

In the invention and the specification, the “hexagonal ferrite powder”is a ferromagnetic powder in which a hexagonal ferrite type crystalstructure is detected as a main phase by X-ray diffraction analysis. Themain phase is a structure to which a diffraction peak at the highestintensity in an X-ray diffraction spectrum obtained by the X-raydiffraction analysis belongs. For example, in a case where thediffraction peak at the highest intensity in the X-ray diffractionspectrum obtained by the X-ray diffraction analysis belongs to ahexagonal ferrite type crystal structure, it is determined that thehexagonal ferrite type crystal structure is detected as a main phase. Ina case where only a single structure is detected by the X-raydiffraction analysis, this detected structure is set as a main phase.The hexagonal ferrite type crystal structure includes at least an ironatom, a divalent metal atom, and an oxygen atom as constituting atoms. Adivalent metal atom is a metal atom which can be divalent cations asions, and examples thereof include an alkali earth metal atom such as astrontium atom, a barium atom, or a calcium atom, and a lead atom. Inthe invention and the specification, the hexagonal strontium ferritepowder is powder in which a main divalent metal atom included in thispowder is a strontium atom, and the hexagonal barium ferrite powder is apowder in which a main divalent metal atom included in this powder is abarium atom. The main divalent metal atom is a divalent metal atomoccupying the greatest content in the divalent metal atom included inthe powder based on atom %. However, the divalent metal atom describedabove does not include rare earth atom. The “rare earth atom” of theinvention and the specification is selected from the group consisting ofa scandium atom (Sc), an yttrium atom (Y), and a lanthanoid atom. Thelanthanoid atom is selected from the group consisting of a lanthanumatom (La), a cerium atom (Ce), a praseodymium atom (Pr), a neodymiumatom (Nd), a promethium atom (Pm), a samarium atom (Sm), an europiumatom (Eu), a gadolinium atom (Gd), a terbium atom (Tb), a dysprosiumatom (Dy), a holmium atom (Ho), an erbium atom (Er), a thulium atom(Tm), an ytterbium atom (Yb), and a lutetium atom (Lu).

Hereinafter, the hexagonal strontium ferrite powder which is one aspectof the hexagonal ferrite powder will be described more specifically.

An activation volume of the hexagonal strontium ferrite powder ispreferably 800 to 1600 nm³. The atomized hexagonal strontium ferritepowder showing the activation volume in the range described above issuitable for manufacturing a magnetic recording medium exhibitingexcellent electromagnetic conversion characteristics. The activationvolume of the hexagonal strontium ferrite powder is preferably equal toor greater than 800 nm³, and can also be, for example, equal to orgreater than 850 nm³. In addition, from a viewpoint of further improvingthe electromagnetic conversion characteristics, the activation volume ofthe hexagonal strontium ferrite powder is more preferably equal to orsmaller than 1500 nm³, even more preferably equal to or smaller than1400 nm³, still preferably equal to or smaller than 1300 nm³, still morepreferably equal to or smaller than 1200 nm³, and still even morepreferably equal to or smaller than 1100 nm³. The same applies to theactivation volume of the hexagonal barium ferrite powder.

The “activation volume” is a unit of magnetization reversal and an indexshowing a magnetic magnitude of the particles. Regarding the activationvolume and an anisotropy constant Ku which will be described laterdisclosed in the invention and the specification, magnetic field sweeprates of a coercivity Hc measurement part at time points of 3 minutesand 30 minutes are measured by using an oscillation sample typemagnetic-flux meter (measurement temperature: 23° C.±1° C.), and theactivation volume and the anisotropy constant Ku are values acquiredfrom the following relational expression of Hc and an activation volumeV. A unit of the anisotropy constant Ku is 1 erg/cc=1.0×10⁻¹ J/m³.Hc=2Ku/Ms{1−[(kT/KuV)ln(At/0.693)]^(1/2)}

[In the expression, Ku: anisotropy constant (unit: J/m³), Ms: saturationmagnetization (unit: kA/m), k: Boltzmann's constant, T: absolutetemperature (unit: K), V: activation volume (unit: cm³), A: spinprecession frequency (unit: s⁻¹), and t: magnetic field reversal time(unit: s)]

The anisotropy constant Ku can be used as an index of reduction ofthermal fluctuation, that is, improvement of thermal stability. Thehexagonal strontium ferrite powder can preferably have Ku equal to orgreater than 1.8×105 J/m³, and more preferably have Ku equal to orgreater than 2.0×105 J/m³. In addition, Ku of the hexagonal strontiumferrite powder can be, for example, equal to or smaller than 2.5×105J/m³. However, the high Ku is preferable, because it means high thermalstability, and thus, Ku is not limited to the exemplified value.

The hexagonal strontium ferrite powder may or may not include the rareearth atom. In a case where the hexagonal strontium ferrite powderincludes the rare earth atom, a content (bulk content) of the rare earthatom is preferably 0.5 to 5.0 atom % with respect to 100 atom % of theiron atom. In one aspect, the hexagonal strontium ferrite powderincluding the rare earth atom can have rare earth atom surface layerportion uneven distribution. The “rare earth atom surface layer portionuneven distribution” of the invention and the specification means that acontent of rare earth atom with respect to 100 atom % of iron atom in asolution obtained by partially dissolving the hexagonal strontiumferrite powder with acid (hereinafter, referred to as a “rare earth atomsurface layer portion content” or simply a “surface layer portioncontent” regarding the rare earth atom) and a content of rare earth atomwith respect to 100 atom % of iron atom in a solution obtained bytotally dissolving the hexagonal strontium ferrite powder with acid(hereinafter, referred to as a “rare earth atom bulk content” or simplya “bulk content” regarding the rare earth atom) satisfy a ratio of rareearth atom surface layer portion content/rare earth atom bulkcontent>1.0.

The content of rare earth atom of the hexagonal strontium ferrite powderwhich will be described later is identical to the rare earth atom bulkcontent. With respect to this, the partial dissolving using acid is todissolve the surface layer portion of particles configuring thehexagonal strontium ferrite powder, and accordingly, the content of rareearth atom in the solution obtained by the partial dissolving is thecontent of rare earth atom in the surface layer portion of the particlesconfiguring the hexagonal strontium ferrite powder. The rare earth atomsurface layer portion content satisfying a ratio of “rare earth atomsurface layer portion content/rare earth atom bulk content>1.0” meansthat the rare earth atoms are unevenly distributed in the surface layerportion (that is, a larger amount of the rare earth atoms is present,compared to that inside), among the particles configuring the hexagonalstrontium ferrite powder. The surface layer portion of the invention andthe specification means a part of the region of the particlesconfiguring the hexagonal strontium ferrite powder from the inside fromthe surface.

In a case where the hexagonal strontium ferrite powder includes the rareearth atom, a content (bulk content) of the rare earth atom ispreferably 0.5 to 5.0 atom % with respect to 100 atom % of the ironatom. It is thought that the hexagonal strontium ferrite powderincluding the rare earth atom having the bulk content in the rangedescribed above and uneven distribution of the rare earth atom in thesurface layer portion of the particles configuring the hexagonalstrontium ferrite powder contribute to the prevention of reduction ofreproduction output during the repeated reproduction. It is surmisedthat this is because the anisotropy constant Ku can be increased due tothe hexagonal strontium ferrite powder including the rare earth atomhaving the bulk content in the range described above and unevendistribution of the rare earth atom in the surface layer portion of theparticles configuring the hexagonal strontium ferrite powder. As thevalue of the anisotropy constant Ku is high, occurrence of a phenomenon,so-called thermal fluctuation can be prevented (that is, thermalstability can be improved). By preventing the occurrence of thermalfluctuation, it is possible to prevent reduction of the reproductionoutput during the repeated reproduction. It is surmised that, the unevendistribution of the rare earth atom in the particle surface layerportion of the hexagonal strontium ferrite powder contributes tostabilization of a spin at an iron (Fe) site in a crystal lattice of thesurface layer portion, thereby increasing the anisotropy constant Ku.

It is surmised that the use of the hexagonal strontium ferrite powderhaving the rare earth atom surface layer portion uneven distribution asthe ferromagnetic powder of the magnetic layer contributes to theprevention of chipping of the surface of the magnetic layer due to thesliding with the magnetic head. That is, it is surmised that thehexagonal strontium ferrite powder having the rare earth atom surfacelayer portion uneven distribution also contributes to the improvement ofrunning durability of the magnetic recording medium. It is surmised thatthis is because the uneven distribution of the rare earth atom on thesurface of the particles configuring the hexagonal strontium ferritepowder contributes to improvement of an interaction between the surfaceof the particles and an organic substance (for example, binding agentand/or additive) included in the magnetic layer, thereby improvinghardness of the magnetic layer.

From a viewpoint of further preventing the reproduction output in therepeated reproduction and/or a viewpoint of further improving runningdurability, the content of rare earth atom (bulk content) is morepreferably 0.5 to 4.5 atom %, even more preferably 1.0 to 4.5 atom %,and still preferably 1.5 to 4.5 atom %.

The bulk content is a content obtained by totally dissolving thehexagonal strontium ferrite powder. In the invention and thespecification, the content of the atom is a bulk content obtained bytotally dissolving the hexagonal strontium ferrite powder, unlessotherwise noted. The hexagonal strontium ferrite powder including therare earth atom may include only one kind of rare earth atom or mayinclude two or more kinds of rare earth atom, as the rare earth atom. Ina case where two or more kinds of rare earth atom are included, the bulkcontent is obtained from the total of the two or more kinds of rareearth atom. The same also applies to the other components of theinvention and the specification. That is, for a given component, onlyone kind may be used or two or more kinds may be used, unless otherwisenoted. In a case where two or more kinds are used, the content is acontent of the total of the two or more kinds.

In a case where the hexagonal strontium ferrite powder includes the rareearth atom, the rare earth atom included therein may be any one or morekinds of the rare earth atom. Examples of the rare earth atom preferablefrom a viewpoint of further preventing reduction of the reproductionoutput during the repeated reproduction include a neodymium atom, asamarium atom, an yttrium atom, and a dysprosium atom, a neodymium atom,a samarium atom, an yttrium atom are more preferable, and a neodymiumatom is even more preferable.

In the hexagonal strontium ferrite powder having the rare earth atomsurface layer portion uneven distribution, a degree of unevendistribution of the rare earth atom is not limited, as long as the rareearth atom is unevenly distributed in the surface layer portion of theparticles configuring the hexagonal strontium ferrite powder. Forexample, regarding the hexagonal strontium ferrite powder having therare earth atom surface layer portion uneven distribution, a ratio ofthe surface layer portion content of the rare earth atom obtained bypartial dissolving performed under the dissolving conditions which willbe described later and the bulk content of the rare earth atom obtainedby total dissolving performed under the dissolving conditions which willbe described later, “surface layer portion content/bulk content” isgreater than 1.0 and can be equal to or greater than 1.5. The “surfacelayer portion/bulk content” greater than 1.0 means that the rare earthatom is unevenly distributed in the surface layer portion (that is, alarger amount of the rare earth atoms is present, compared to thatinside), among the particles configuring the hexagonal strontium ferritepowder. In addition, the ratio of the surface layer portion content ofthe rare earth atom obtained by partial dissolving performed under thedissolving conditions which will be described later and the bulk contentof the rare earth atom obtained by total dissolving performed under thedissolving conditions which will be described later, “surface layerportion content/bulk content” can be, for example, equal to or smallerthan 10.0, equal to or smaller than 9.0, equal to or smaller than 8.0,equal to or smaller than 7.0, equal to or smaller than 6.0, equal to orsmaller than 5.0, or equal to or smaller than 4.0. However, in thehexagonal strontium ferrite powder having the rare earth atom surfacelayer portion uneven distribution, the “surface layer portioncontent/bulk content” is not limited to the exemplified upper limit orthe lower limit, as long as the rare earth atom is unevenly distributedin the surface layer portion of the particles configuring the hexagonalstrontium ferrite powder.

The partial dissolving and the total dissolving of the hexagonalstrontium ferrite powder will be described below. Regarding thehexagonal strontium ferrite powder present as the powder, sample powderfor the partial dissolving and the total dissolving are collected frompowder of the same batch. Meanwhile, regarding the hexagonal strontiumferrite powder included in a magnetic layer of a magnetic recordingmedium, a part of the hexagonal strontium ferrite powder extracted fromthe magnetic layer is subjected to the partial dissolving and the otherpart is subjected to the total dissolving. The extraction of thehexagonal strontium ferrite powder from the magnetic layer can beperformed by a method disclosed in a paragraph 0032 of JP2015-091747A.

The partial dissolving means dissolving performed so that the hexagonalstrontium ferrite powder remaining in the solution can be visuallyconfirmed at the time of the completion of the dissolving. For example,by performing the partial dissolving, a region of the particlesconfiguring the hexagonal strontium ferrite powder which is 10% to 20%by mass with respect to 100% by mass of a total of the particles can bedissolved. On the other hand, the total dissolving means dissolvingperformed until the hexagonal strontium ferrite powder remaining in thesolution is not visually confirmed at the time of the completion of thedissolving.

The partial dissolving and the measurement of the surface layer portioncontent are, for example, performed by the following method. However,dissolving conditions such as the amount of sample powder and the likedescribed below are merely examples, and dissolving conditions capableof performing the partial dissolving and the total dissolving can berandomly used.

A vessel (for example, beaker) containing 12 mg of sample powder and 10ml of hydrochloric acid having a concentration of 1 mol/L is held on ahot plate at a set temperature of 70° C. for 1 hour. The obtainedsolution is filtered with a membrane filter having a hole diameter of0.1 μm. The element analysis of the filtrate obtained as described aboveis performed by an inductively coupled plasma (ICP) analysis device. Bydoing so, the rare earth atom surface layer portion content with respectto 100 atom % of the iron atom can be obtained. In a case where aplurality of kinds of rare earth atoms are detected from the elementanalysis, a total content of the entirety of the rare earth atoms is thesurface layer portion content. The same applies to the measurement ofthe bulk content.

Meanwhile, the total dissolving and the measurement of the bulk contentare, for example, performed by the following method.

A vessel (for example, beaker) containing 12 mg of sample powder and 10ml of hydrochloric acid having a concentration of 4 mol/L is held on ahot plate at a set temperature of 80° C. for 3 hours. After that, theprocess is performed in the same manner as in the partial dissolving andthe measurement of the surface layer portion content, and the bulkcontent with respect to 100 atom % of the iron atom can be obtained.

From a viewpoint of increasing reproducing output in a case ofreproducing data recorded on a magnetic recording medium, it isdesirable that the mass magnetization σs of ferromagnetic powderincluded in the magnetic recording medium is high. In regards to thispoint, in hexagonal strontium ferrite powder which includes the rareearth atom but does not have the rare earth atom surface layer portionuneven distribution, σs tends to significantly decrease, compared tothat in hexagonal strontium ferrite powder not including the rare earthatom. With respect to this, it is thought that the hexagonal strontiumferrite powder having the rare earth atom surface layer portion unevendistribution is preferable for preventing such a significant decrease inσs. In one aspect, σs of the hexagonal strontium ferrite powder can beequal to or greater than 45 A·m²/kg and can also be equal to or greaterthan 47 A·m²/kg. On the other hand, from a viewpoint of noise reduction,σs is preferably equal to or smaller than 80 A·m²/kg and more preferablyequal to or smaller than 60 A·m²/kg. σs can be measured by using awell-known measurement device capable of measuring magnetic propertiessuch as an oscillation sample type magnetic-flux meter. In the inventionand the specification, the mass magnetization σs is a value measured ata magnetic field strength of 15 kOe, unless otherwise noted. 1kOe=(10⁶/4π) A/m

Regarding the content (bulk content) of the constituting atom in thehexagonal strontium ferrite powder, a content of the strontium atom canbe, for example, 2.0 to 15.0 atom % with respect to 100 atom % of theiron atom. In one aspect, in the hexagonal strontium ferrite powder, thedivalent metal atom included in this powder can be only a strontiumatom. In another aspect, the hexagonal strontium ferrite powder can alsoinclude one or more kinds of other divalent metal atoms, in addition tothe strontium atom. For example, a barium atom and/or a calcium atom canbe included. In a case where the other divalent metal atom other thanthe strontium atom is included, a content of a barium atom and a contentof a calcium atom in the hexagonal strontium ferrite powder respectivelycan be, for example, 0.05 to 5.0 atom % with respect to 100 atom % ofthe iron atom.

As the crystal structure of the hexagonal ferrite, a magnetoplumbitetype (also referred to as an “M type”), a W type, a Y type, and a Z typeare known. The hexagonal strontium ferrite powder may have any crystalstructure. The crystal structure can be confirmed by X-ray diffractionanalysis. In the hexagonal strontium ferrite powder, a single crystalstructure or two or more kinds of crystal structure can be detected bythe X-ray diffraction analysis. For example, in one aspect, in thehexagonal strontium ferrite powder, only the M type crystal structurecan be detected by the X-ray diffraction analysis. For example, the Mtype hexagonal ferrite is represented by a compositional formula ofAFe₁₂O₁₉. Here, A represents a divalent metal atom, in a case where thehexagonal strontium ferrite powder has the M type, A is only a strontiumatom (Sr), or in a case where a plurality of divalent metal atoms areincluded as A, the strontium atom (Sr) occupies the hexagonal strontiumferrite powder with the greatest content based on atom % as describedabove. A content of the divalent metal atom in the hexagonal strontiumferrite powder is generally determined according to the type of thecrystal structure of the hexagonal ferrite and is not particularlylimited. The same applies to a content of an iron atom and a content ofan oxygen atom. The hexagonal strontium ferrite powder at least includesan iron atom, a strontium atom, and an oxygen atom, and can furtherinclude a rare earth atom. In addition, the hexagonal strontium ferritepowder may or may not include atoms other than these atoms. As anexample the hexagonal strontium ferrite powder may include an aluminumatom (Al). A content of the aluminum atom can be, for example, 0.5 to10.0 atom % with respect to 100 atom % of the iron atom. From aviewpoint of further preventing the reduction of the reproducing outputduring the repeated reproduction, the hexagonal strontium ferrite powderincludes the iron atom, the strontium atom, the oxygen atom, and therare earth atom, and a content of the atoms other than these atoms ispreferably equal to or smaller than 10.0 atom %, more preferably 0 to5.0 atom %, and may be 0 atom % with respect to 100 atom % of the ironatom. That is, in one aspect, the hexagonal strontium ferrite powder maynot include atoms other than the iron atom, the strontium atom, theoxygen atom, and the rare earth atom. The content shown with atom %described above is obtained by converting a value of the content (unit:% by mass) of each atom obtained by totally dissolving the hexagonalstrontium ferrite powder into a value shown as atom % by using theatomic weight of each atom. In addition, in the invention and thespecification, a given atom which is “not included” means that thecontent thereof obtained by performing total dissolving and measurementby using an ICP analysis device is 0% by mass. A detection limit of theICP analysis device is generally equal to or smaller than 0.01 ppm(parts per million) based on mass. The expression “not included” is usedas a meaning including that a given atom is included with the amountsmaller than the detection limit of the ICP analysis device. In oneaspect, the hexagonal strontium ferrite powder does not include abismuth atom (Bi).

Metal Powder

As a preferred specific example of the ferromagnetic powder,ferromagnetic metal powder can also be used. For details of theferromagnetic metal powder, descriptions disclosed in paragraphs 0137 to0141 of JP2011-216149A and paragraphs 0009 to 0023 of JP2005-251351A canbe referred to, for example.

ε-Iron Oxide Powder

As a preferred specific example of the ferromagnetic powder, an ε-ironoxide powder can also be used. In the invention and the specification,the “ε-iron oxide powder” is a ferromagnetic powder in which an ε-ironoxide type crystal structure is detected as a main phase by X-raydiffraction analysis. For example, in a case where the diffraction peakat the highest intensity in the X-ray diffraction spectrum obtained bythe X-ray diffraction analysis belongs to an ε-iron type crystalstructure, it is determined that the ε-iron type crystal structure isdetected as a main phase. As a producing method of the ε-iron oxidepowder, a producing method from a goethite, and a reverse micelle methodare known. All of the producing methods is well known. For example, fora method of producing the ε-iron oxide powder in which a part of Fe issubstituted with a substitutional atom such as Ga, Co, Ti, Al, or Rh, adescription disclosed in J. Jpn. Soc. Powder Metallurgy Vol. 61Supplement, No. 51, pp. S280-S284, J. Mater. Chem. C, 2013, 1, pp.5200-5206 can be referred to, for example. However, the producing methodof the ε-iron oxide powder which can be used as the ferromagnetic powderin the magnetic layer of the magnetic recording medium is not limited tothe method described here.

The activation volume of the ε-iron oxide powder is preferably in arange of 300 to 1500 nm³. The atomized ε-iron oxide powder showing theactivation volume in the range described above is suitable formanufacturing a magnetic recording medium exhibiting excellentelectromagnetic conversion characteristics. The activation volume of theε-iron oxide powder is preferably equal to or greater than 300 nm³, andcan also be, for example, equal to or greater than 500 nm³. In addition,from a viewpoint of further improving the electromagnetic conversioncharacteristics, the activation volume of the ε-iron oxide powder ismore preferably equal to or smaller than 1400 nm³, even more preferablyequal to or smaller than 1300 nm³, still preferably equal to or smallerthan 1200 nm³, and still more preferably equal to or smaller than 1100nm³.

The anisotropy constant Ku can be used as an index of reduction ofthermal fluctuation, that is, improvement of thermal stability. Theε-iron oxide powder can preferably have Ku equal to or greater than3.0×10⁴ J/m³, and more preferably have Ku equal to or greater than8.0×10⁴ J/m³. In addition, Ku of the ε-iron oxide powder can be, forexample, equal to or smaller than 3.0×10⁵ J/m³. However, the high Ku ispreferable, because it means high thermal stability, and thus, Ku is notlimited to the exemplified value.

From a viewpoint of increasing reproducing output in a case ofreproducing data recorded on a magnetic recording medium, it isdesirable that the mass magnetization σs of ferromagnetic powderincluded in the magnetic recording medium is high. In regard to thispoint, in one aspect, σs of the ε-iron oxide powder can be equal to orgreater than 8 A·m²/kg and can also be equal to or greater than 12A·m²/kg. On the other hand, from a viewpoint of noise reduction, σs ofthe ε-iron oxide powder is preferably equal to or smaller than 40A·m²/kg and more preferably equal to or smaller than 35 A·m²/kg.

In the invention and the specification, average particle sizes ofvarious powder such as the ferromagnetic powder and the like are valuesmeasured by the following method with a transmission electronmicroscope, unless otherwise noted.

The powder is imaged at an imaging magnification ratio of 100,000 with atransmission electron microscope, the image is printed on photographicprinting paper or displayed on a display so that the total magnificationratio of 500,000 to obtain an image of particles configuring the powder.A target particle is selected from the obtained image of particles, anoutline of the particle is traced with a digitizer, and a size of theparticle (primary particle) is measured. The primary particle is anindependent particle which is not aggregated.

The measurement described above is performed regarding 500 particlesrandomly extracted. An arithmetical mean of the particle size of 500particles obtained as described above is an average particle size of thepowder. As the transmission electron microscope, a transmission electronmicroscope H-9000 manufactured by Hitachi, Ltd. can be used, forexample. In addition, the measurement of the particle size can beperformed by well-known image analysis software, for example, imageanalysis software KS-400 manufactured by Carl Zeiss. The averageparticle size shown in examples which will be described later is a valuemeasured by using transmission electron microscope H-9000 manufacturedby Hitachi, Ltd. as the transmission electron microscope, and imageanalysis software KS-400 manufactured by Carl Zeiss as the imageanalysis software, unless otherwise noted. In the invention and thespecification, the powder means an aggregate of a plurality ofparticles. For example, the ferromagnetic powder means an aggregate of aplurality of ferromagnetic particles. The aggregate of a plurality ofparticles is not limited to an aspect in which particles configuring theaggregate directly come into contact with each other, but also includesan aspect in which a binding agent, an additive, or the like which willbe described later is interposed between the particles. A term,particles may be used for representing the powder.

As a method of collecting a sample powder from the magnetic recordingmedium in order to measure the particle size, a method disclosed in aparagraph 0015 of JP2011-048878A can be used, for example.

In the invention and the specification, unless otherwise noted, (1) in acase where the shape of the particle observed in the particle imagedescribed above is a needle shape, a fusiform shape, or a columnar shape(here, a height is greater than a maximum long diameter of a bottomsurface), the size (particle size) of the particles configuring thepowder is shown as a length of a long axis configuring the particle,that is, a long axis length, (2) in a case where the shape of theparticle is a planar shape or a columnar shape (here, a thickness or aheight is smaller than a maximum long diameter of a plate surface or abottom surface), the particle size is shown as a maximum long diameterof the plate surface or the bottom surface, and (3) in a case where theshape of the particle is a sphere shape, a polyhedron shape, or anunspecified shape, and the long axis configuring the particles cannot bespecified from the shape, the particle size is shown as an equivalentcircle diameter. The equivalent circle diameter is a value obtained by acircle projection method.

In addition, regarding an average acicular ratio of the powder, a lengthof a short axis, that is, a short axis length of the particles ismeasured in the measurement described above, a value of (long axislength/short axis length) of each particle is obtained, and anarithmetical mean of the values obtained regarding 500 particles iscalculated. Here, unless otherwise noted, in a case of (1), the shortaxis length as the definition of the particle size is a length of ashort axis configuring the particle, in a case of (2), the short axislength is a thickness or a height, and in a case of (3), the long axisand the short axis are not distinguished, thus, the value of (long axislength/short axis length) is assumed as 1, for convenience.

In addition, unless otherwise noted, in a case where the shape of theparticle is specified, for example, in a case of definition of theparticle size (1), the average particle size is an average long axislength, in a case of the definition (2), the average particle size is anaverage plate diameter. In a case of the definition (3), the averageparticle size is an average diameter (also referred to as an averageparticle diameter).

The content (filling percentage) of the ferromagnetic powder of themagnetic layer is preferably 50% to 90% by mass and more preferably 60%to 90% by mass. A high filling percentage of the ferromagnetic powder inthe magnetic layer is preferable from a viewpoint of improvement ofrecording density.

Binding Agent and Curing Agent

The magnetic recording medium can be a coating type magnetic recordingmedium, and can include a binding agent in the magnetic layer. Thebinding agent is one or more kinds of resin. As the binding agent,various resins generally used as the binding agent of the coating typemagnetic recording medium can be used. For example, as the bindingagent, a resin selected from a polyurethane resin, a polyester resin, apolyamide resin, a vinyl chloride resin, an acrylic resin obtained bycopolymerizing styrene, acrylonitrile, or methyl methacrylate, acellulose resin such as nitrocellulose, an epoxy resin, a phenoxy resin,and a polyvinylalkylal resin such as polyvinyl acetal or polyvinylbutyral can be used alone or a plurality of resins can be mixed witheach other to be used. Among these, a polyurethane resin, an acrylicresin, a cellulose resin, and a vinyl chloride resin are preferable. Theresin may be a homopolymer or a copolymer. These resins can be used asthe binding agent even in the back coating layer and/or a non-magneticlayer which will be described later.

For the binding agent described above, description disclosed inparagraphs 0028 to 0031 of JP2010-024113A can be referred to. An averagemolecular weight of the resin used as the binding agent can be, forexample, 10,000 to 200,000 as a weight-average molecular weight. Theweight-average molecular weight of the invention and the specificationis a value obtained by performing polystyrene conversion of a valuemeasured by gel permeation chromatography (GPC) under the followingmeasurement conditions. The weight-average molecular weight of thebinding agent shown in examples which will be described later is a valueobtained by performing polystyrene conversion of a value measured underthe following measurement conditions.

GPC device: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8mmID (inner diameter)

30.0 cm)

Eluent: Tetrahydrofuran (THF)

In an embodiment, as the binding agent, a binding agent including anactive hydrogen-containing group can be used. The “activehydrogen-containing group” in the invention and the specification is afunctional group capable of forming a crosslinked structure by a curingreaction of this group with a curable functional group and desorption ofhydrogen atoms included in this group. Examples of the activehydrogen-containing group include a hydroxy group, an amino group(preferably a primary amino group or a secondary amino group), amercapto group, and a carboxy group, a hydroxy group, an amino group anda mercapto group are preferable, and a hydroxy group is more preferable.A concentration of the active hydrogen-containing group in the bindingagent including the active hydrogen-containing group is preferably 0.10meq/g to 2.00 meq/g. eq indicates equivalent and is a unit notconvertible into SI unit. In addition, the concentration of the activehydrogen-containing group can also be shown with a unit “mgKOH/g”. In anembodiment, the concentration of the active hydrogen-containing group inthe resin including the active hydrogen-containing group is preferably 1to 20 mgKOH/g.

In one aspect, as the binding agent, a binding agent including an acidicgroup can be used. The “acidic group” of the invention and thespecification is used as a meaning including a state of a group capableof emitting H⁺ in water or a solvent including water (aqueous solvent)to dissociate anions and salt thereof. Specific examples of the acidicgroup include a sulfonic acid group (—SO₃H), a sulfuric acid group(—OSO₃H), a carboxy group, a phosphoric acid group, and salt thereof.For example, salt of sulfonic acid group (—SO₃H) is represented by—SO₃M, and M represents a group representing an atom (for example,alkali metal atom or the like) which may be cations in water or in anaqueous solvent. The same applies to aspects of salt of various groupsdescribed above. As an example of the binding agent including the acidicgroup, a resin including at least one kind of acidic group selected fromthe group consisting of a sulfonic acid group and salt thereof (forexample, a polyurethane resin or a vinyl chloride resin) can be used.However, the resin included in the magnetic layer is not limited tothese resins. In addition, in the binding agent including the acidicgroup, a content of the acidic group can be, for example, 0.03 to 0.50meq/g. The content of various functional groups such as the acidic groupincluded in the resin can be obtained by a well-known method inaccordance with the kind of the functional group. The amount of thebinding agent used in a magnetic layer forming composition can be, forexample, 1.0 to 30.0 parts by mass with respect to 100.0 parts by massof the ferromagnetic powder.

In addition, a curing agent can also be used together with the resinwhich can be used as the binding agent. As the curing agent, in oneaspect, a thermosetting compound which is a compound in which a curingreaction (crosslinking reaction) proceeds due to heating can be used,and in another aspect, a photocurable compound in which a curingreaction (crosslinking reaction) proceeds due to light irradiation canbe used. At least a part of the curing agent is included in the magneticlayer in a state of being reacted (crosslinked) with other componentssuch as the binding agent, by proceeding the curing reaction in themagnetic layer forming step. This point is the same as regarding a layerformed by using a composition, in a case where the composition used forforming the other layer includes the curing agent. The preferred curingagent is a thermosetting compound, polyisocyanate is suitable. Fordetails of the polyisocyanate, descriptions disclosed in paragraphs 0124and 0125 of JP2011-216149A can be referred to, for example. The amountof the curing agent can be, for example, 0 to 80.0 parts by mass withrespect to 100.0 parts by mass of the binding agent in the magneticlayer forming composition, and is preferably 50.0 to 80.0 parts by mass,from a viewpoint of improvement of hardness of the magnetic layer.

The description regarding the binding agent and the curing agentdescribed above can also be applied to the back coating layer and/or thenon-magnetic layer. In this case, the description regarding the contentcan be applied by replacing the ferromagnetic powder with thenon-magnetic powder.

Additives

The magnetic layer includes may include one or more kinds of additives,if necessary. As the additives, the curing agent described above is usedas an example. In addition, examples of the additive included in themagnetic layer include non-magnetic powder (for example, inorganicpowder or carbon black), a lubricant, a dispersing agent, a dispersingassistant, an antibacterial agent, an antistatic agent, and anantioxidant. As the non-magnetic powder, non-magnetic powder which canfunction as an abrasive, non-magnetic powder (for example, non-magneticcolloid particles) which can function as a projection formation agentwhich forms projections suitably protruded from the surface of themagnetic layer, and the like can be used. An average particle size ofcolloidal silica (silica colloid particles) shown in the examples whichwill be described later is a value obtained by a method disclosed in ameasurement method of an average particle diameter in a paragraph 0015of JP2011-048878A. As the additives, a commercially available productcan be suitably selected according to the desired properties ormanufactured by a well-known method, and can be used with any amount. Asan example of the additive which can be used in the magnetic layerincluding the abrasive, a dispersing agent disclosed in paragraphs 0012to 0022 of JP2013-131285A can be used as a dispersing agent forimproving dispersibility of the abrasive. For example, for thelubricant, a description disclosed in paragraphs 0030 to 0033, 0035, and0036 of JP2016-126817A can be referred to. The non-magnetic layer mayinclude the lubricant. For the lubricant which may be included in thenon-magnetic layer, a description disclosed in paragraphs 0030, 0031,0034, 0035, and 0036 of JP2016-126817A can be referred to. For thedispersing agent, a description disclosed in paragraphs 0061 and 0071 ofJP2012-133837A can be referred to. In addition, for the dispersingagent, a description disclosed in paragraphs 0061 and 0071 ofJP2012-133837, and paragraph 0035 of JP2017-016721A can also be referredto. For the additive of the magnetic layer, a description disclosed inparagraphs 0035 to 0077 of JP2016-051493A can also be referred to.

The dispersing agent may be included in the non-magnetic layer. For thedispersing agent which may be included in the non-magnetic layer, adescription disclosed in a paragraph 0061 of JP2012-133837A can bereferred to.

As various additives, a commercially available product can be suitablyselected according to the desired properties or manufactured by awell-known method, and can be used with any amount.

The magnetic layer described above can be provided on the surface of thenon-magnetic support directly or indirectly through the non-magneticlayer.

Non-Magnetic Layer

Next, the non-magnetic layer will be described. The magnetic recordingmedium may include a magnetic layer directly on the non-magnetic supportor may include a non-magnetic layer including the non-magnetic powderbetween the non-magnetic support and the magnetic layer. Thenon-magnetic powder used in the non-magnetic layer may be powder of aninorganic substance or powder of an organic substance. In addition,carbon black and the like can be used. Examples of the inorganicsubstance include metal, metal oxide, metal carbonate, metal sulfate,metal nitride, metal carbide, and metal sulfide. These non-magneticpowder can be purchased as a commercially available product or can bemanufactured by a well-known method. For details thereof, descriptionsdisclosed in paragraphs 0146 to 0150 of JP2011-216149A can be referredto. For carbon black capable of being used in the non-magnetic layer, adescription disclosed in paragraphs 0040 and 0041 of JP2010-024113A canbe referred to. The content (filling percentage) of the non-magneticpowder of the non-magnetic layer is preferably 50% to 90% by mass andmore preferably 60% to 90% by mass.

The non-magnetic layer can include a binding agent and can also includeadditives. In regards to details of a binding agent or additives of thenon-magnetic layer, the well-known technology regarding the non-magneticlayer can be applied. In addition, in regards to the type and thecontent of the binding agent, and the type and the content of theadditive, for example, the well-known technology regarding the magneticlayer can be applied.

The non-magnetic layer of the invention and the specification alsoincludes a substantially non-magnetic layer including a small amount offerromagnetic powder as impurities or intentionally, together with thenon-magnetic powder. Here, the substantially non-magnetic layer is alayer having a residual magnetic flux density equal to or smaller than10 mT, a layer having coercivity equal to or smaller than 100 Oe, or alayer having a residual magnetic flux density equal to or smaller than10 mT and coercivity equal to or smaller than 100 Oe. It is preferablethat the non-magnetic layer does not have a residual magnetic fluxdensity and coercivity.

Back Coating Layer

The magnetic recording medium includes a back coating layer including anon-magnetic powder on a surface of the non-magnetic support opposite tothe surface provided with the magnetic layer. Regarding the kind of thenon-magnetic powder included in the back coating layer, the descriptionregarding the non-magnetic powder included in the non-magnetic layer canbe referred to. The non-magnetic powder included in the back coatinglayer can be preferably one or more kinds of non-magnetic powderselected from the group consisting of inorganic powder and carbon black.For example, the magnetic recording medium can include an inorganicpowder, as main powder of the non-magnetic powder in the back coatinglayer (non-magnetic powder, the largest amount of which is includedbased on mass, among the non-magnetic powder). In a case where thenon-magnetic powder included in the back coating layer is one or morekinds of the non-magnetic powder selected from the group consisting ofthe inorganic powder and carbon black, a percentage of the inorganicpowder with respect to 100.0 parts by mass of a total amount of thenon-magnetic powder can be, for example, greater than 50.0 parts by massand equal to or smaller than 100.0 parts by mass and more preferably60.0 parts by mass to 100.0 parts by mass. In a case where thepercentage of the inorganic powder in the non-magnetic powder isincreased, the value of the difference (S_(after)−S_(before)) tends todecrease in some cases.

An average particle size of the non-magnetic powder can be, for example,10 to 200 nm. An average particle size of the inorganic powder ispreferably 50 to 200 nm and more preferably 80 to 150 nm. Meanwhile, anaverage particle size of the carbon black is preferably 10 to 50 nm andmore preferably 15 to 30 nm.

The back coating layer can include a binding agent and can also includeadditives. As an example of the additive, a well-known dispersing agentwhich can contribute to improvement in dispersibility of thenon-magnetic powder can be used.

In addition, as an example of the additive, a lubricant is also used.

For example, as the lubricant, fatty acid, fatty acid ester, and fattyacid amide can be used, and a magnetic layer can be formed by using oneor more kinds selected from the group consisting of fatty acid, fattyacid ester, and fatty acid amide.

Examples of fatty acid include lauric acid, myristic acid, palmiticacid, stearic acid, oleic acid, linoleic acid, linolenic acid, behenicacid, erucic acid, and elaidic acid, stearic acid, myristic acid, andpalmitic acid are preferable, and stearic acid is more preferable. Fattyacid may be included in the magnetic layer in a state of salt such asmetal salt.

As fatty acid ester, esters of lauric acid, myristic acid, palmiticacid, stearic acid, oleic acid, linoleic acid, linolenic acid, behenicacid, erucic acid, and elaidic acid can be used, for example. Specificexamples thereof include butyl myristate, butyl palmitate, butylstearate, neopentyl glycol dioleate, sorbitan monostearate, sorbitandistearate, sorbitan tristearate, oleyl oleate, isocetyl stearate,isotridecyl stearate, octyl stearate, isooctyl stearate, amyl stearate,and butoxyethyl stearate.

As fatty acid amide, amide of various fatty acid described above isused, and examples thereof include lauric acid amide, myristic acidamide, palmitic acid amide, and stearic acid amide.

A content of fatty acid in the back coating layer is, for example, 0 to10.0 parts by mass, preferably 0.1 to 10.0 parts by mass, and morepreferably 1.0 to 7.0 parts by mass with respect to 100.0 parts by massof non-magnetic powder included in the back coating layer. A content offatty acid ester in the back coating layer is, for example, 0.1 to 10.0parts by mass and preferably 1.0 to 5.0 parts by mass with respect to100.0 parts by mass of non-magnetic powder included in the back coatinglayer. A content of fatty acid amide in the back coating layer is, forexample, 0 to 3.0 parts by mass, preferably 0 to 2.0 parts by mass, andmore preferably 0 to 1.0 part by mass with respect to 100.0 parts bymass of the non-magnetic powder included in the back coating layer.

In the invention and the specification, a given component may be usedalone or used in combination of two or more kinds thereof, unlessotherwise noted. In a case where two or more kinds of given componentsare used, the content is a total content of the two or more kinds ofcomponents.

In regards to the binding agent included in the back coating layer andvarious additives, a well-known technology regarding the back coatinglayer can be applied, and a well-known technology regarding the list ofthe magnetic layer and/or the non-magnetic layer can also be applied.For example, for the back coating layer, descriptions disclosed inparagraphs 0018 to 0020 of JP2006-331625A and page 4, line 65, to page5, line 38, of U.S. Pat. No. 7,029,774B can be referred to.

Various Thicknesses

The thickness of the non-magnetic support of the magnetic recordingmedium is, for example, 3.0 to 80.0 μm, preferably 3.0 to 50.0 μm, andmore preferably 3.0 to 10.0 μm. In a case where the non-magnetic supportis a laminated film of two or more layers, the thickness of thenon-magnetic support is a total thickness of the laminated film.

A thickness of the magnetic layer can be optimized according to theamount of a saturation magnetization of a magnetic head used, a head gaplength, a recording signal band, and the like, and is, for example, 10nm to 100 nm, and is preferably 20 to 90 nm and more preferably 30 to 70nm, from a viewpoint of realization of high-density recording. Themagnetic layer may be at least one layer, or the magnetic layer can beseparated to two or more layers having magnetic properties, and aconfiguration regarding a well-known multilayered magnetic layer can beapplied. A thickness of the magnetic layer which is separated into twoor more layers is a total thickness of the layers.

A thickness of the non-magnetic layer is, for example, equal to orgreater than 50 nm, preferably equal to or greater than 70 nm, and morepreferably equal to or greater than 100 nm. Meanwhile, the thickness ofthe non-magnetic layer is, for example, preferably equal to or smallerthan 800 nm, and more preferably equal to or smaller than 500 nm.

A thickness of the back coating layer is preferably equal to or smallerthan 0.9 μm and even more preferably 0.1 to 0.7 μm.

The thicknesses of various layers and the non-magnetic support of themagnetic recording medium can be obtained by a well-known film thicknessmeasurement method. As an example, a cross section of the magneticrecording medium in a thickness direction is exposed by a well-knownmethod of ion beams or microtome, and the exposed cross section isobserved with a scanning electron microscope. In the cross sectionobservation, various thicknesses can be obtained as the thicknessobtained at any one portion, or as an arithmetical mean of thicknessesobtained at a plurality of portions which are two or more portionsrandomly extracted, for example, two portions. Alternatively, thethickness of each layer may be obtained as a designed thicknesscalculated under the manufacturing conditions.

Manufacturing Method

Preparation of Each Layer Forming Composition

Composition for forming the magnetic layer, the back coating layer, thenon-magnetic layer generally include a solvent, together with thevarious components described above. As the solvent, various organicsolvents generally used for manufacturing a coating type magneticrecording medium can be used. The amount of solvent in each layerforming composition is not particularly limited, and can be identical tothat in each layer forming composition of a typical coating typemagnetic recording medium. A step of preparing the composition forforming each layer generally includes at least a kneading step, adispersing step, and a mixing step provided before and after thesesteps, in a case where necessary. Each step may be divided into two ormore stages. All of raw materials used in the invention may be added atan initial stage or in a middle stage of each step. In addition, eachraw material may be separately added in two or more steps.

In order to prepare each layer forming composition, a well-knowntechnology can be used. In the kneading step, an open kneader, acontinuous kneader, a pressure kneader, or a kneader having a strongkneading force such as an extruder is preferably used. For details ofthe kneading processes, descriptions disclosed in JP1989-106338A(JP-H01-106338A) and JP1989-079274A (JP-H01-079274A) can be referred to.In addition, in order to disperse each layer forming composition, one ormore kinds of dispersion beads selected from the group consisting ofglass beads and other dispersion beads can be used as a dispersionmedium. As such dispersion beads, zirconia beads, titania beads, andsteel beads which are dispersion beads having high specific gravity aresuitable. These dispersion beads may be used by optimizing a particlediameter (bead diameter) and a filling percentage of the dispersionbeads. As a disperser, a well-known disperser can be used. Each layerforming composition may be filtered by a well-known method beforeperforming the coating step. The filtering can be performed by using afilter, for example. As the filter used in the filtering, a filterhaving a hole diameter of 0.01 to 3 μm (for example, filter made ofglass fiber or filter made of polypropylene) can be used, for example.

Coating Step

The magnetic layer can be formed, for example, by directly applying themagnetic layer forming composition onto the non-magnetic support orperforming multilayer coating of the magnetic layer forming compositionwith the non-magnetic layer forming composition in order or at the sametime. The back coating layer can be formed by applying the back coatinglayer forming composition to a side of the non-magnetic support oppositeto a side provided with the magnetic layer (or to be provided with themagnetic layer). For details of the coating for forming each layer, adescription disclosed in a paragraph 0051 of JP2010-024113A can bereferred to.

Other Steps

After the coating step, various processes such as a drying process, analignment process of the magnetic layer, and a surface smoothingtreatment (calender process) can be performed. For various steps, adescription disclosed in paragraphs 0052 to 0057 of JP2010-024113A canbe referred to. For example, a homeotropic alignment process can beperformed by a well-known method such as a method using a differentpolar facing magnet. In the alignment zone, a drying speed of thecoating layer can be controlled by a temperature, an air flow of the dryair and/or a transporting rate in the alignment zone. In addition, thecoating layer may be preliminarily dried before transporting to thealignment zone.

In any stage after the coating step of the back coating layer formingcomposition, the heating process of a coating layer formed by applyingthe back coating layer forming composition is preferably performed. Thisheating process can be performed before and/or after the calenderprocess, for example. The heating process can be, for example, performedby placing a support, on which the coating layer formed by applying theback coating layer forming composition is formed, under heatedatmosphere. The heated atmosphere can be an atmosphere at an atmospheretemperature of 65° C. to 90° C., and more preferably an atmosphere at anatmosphere temperature of 65° C. to 75° C. This atmosphere can be, forexample, the atmosphere. The heating process under the heated atmospherecan be, for example, performed for 20 to 50 hours. In an embodiment, byperforming this heating process, the curing reaction of the curablefunctional group of the curing agent can proceed.

One Aspect of Manufacturing Method

As one aspect of the manufacturing method of the magnetic recordingmedium, a manufacturing method including wiping out the surface of theback coating layer with a wiping material permeated with methyl ethylketone (hereinafter, also referred to as a “methyl ethyl ketone wipingtreatment”), preferably after the heating process can be used. It isthought that the presence of the component capable of being removed bythis methyl ethyl ketone wiping treatment, on the surface of the backcoating layer, promotes the attachment of moisture to the surface of theback coating layer, in a case where a temperature change from a lowtemperature to a high temperature occurs under high humidity. The methylethyl ketone wiping treatment can be performed by using a wipingmaterial permeated with methyl ethyl ketone, instead of a wipingmaterial used in a dry wiping treatment, based on a dry wiping treatmentgenerally performed in the manufacturing step of the magnetic recordingmedium. For example, in the tape-shaped magnetic recording medium(magnetic tape), the methyl ethyl ketone wiping treatment can beperformed on the surface of the back coating layer, by causing themagnetic tape to run between a sending roller and a winding roller,after or before slitting the magnetic tape to have a width accommodatedin a magnetic tape cartridge, and pressing a wiping material (forexample, cloth (for example, non-woven fabric) or paper (for example,tissue paper) permeated with methyl ethyl ketone to the surface of theback coating layer of the magnetic tape during running. A running speedof the magnetic tape during the running and a tension applied in alongitudinal direction of the surface of the back coating layer(hereinafter, simply referred to as a “tension”) can be identical totreatment conditions generally used in the dry wiping treatmentgenerally performed in the manufacturing step of the magnetic recordingmedium. For example, a running speed of the magnetic tape in the methylethyl ketone wiping treatment can be approximately 60 to 600 m/min, andthe tension can be approximately 0.196 to 3.920 N (newton). In addition,the methyl ethyl ketone wiping treatment can be performed at least once.It is preferable to set the treatment conditions and the number of timesof the methyl ethyl ketone wiping treatment so that the spacingdifference (S_(after)−S_(before)) before and after the methyl ethylketone cleaning is greater than 0 nm and equal to or smaller than 30.0nm.

The polishing treatment and/or the dry wiping treatment generallyperformed in the manufacturing step of the coating type magneticrecording medium (hereinafter, these are referred to as a “dry surfacetreatment”) can also be performed one or more times on the surface ofthe back coating layer, before and/or after the methyl ethyl ketonewiping treatment. According to the dry surface treatment, for example,foreign materials which are generated during the manufacturing step suchas scraps generate due to slitting, and attached to the surface of theback coating layer can be removed, for example.

The tape-shaped magnetic recording medium (magnetic tape) has beendescribed above as an example. Various processes can also be performedon a disk-shaped magnetic recording medium (magnetic disk) withreference to the above description.

The magnetic recording medium according to one aspect of the inventiondescribed above can be a magnetic recording medium including an aromaticpolyamide support, in which a deterioration in running stability isslight, even in a case of being stored in a high temperature and highhumidity environment after a temperature change from a low temperatureto a high temperature under high humidity. The temperature change from alow temperature to a high temperature under high humidity is, forexample, a temperature change of approximately 15° C. to 50° C. that isfrom a temperature of higher 0° C. to 15° C. to a temperature of 30° C.to 50° C. in an environment of relative humidity of approximately 70% to100%. The storage in a high temperature and high humidity environmentcan be, for example, storage in an environment of a temperature of 30°C. to 50° C. and relative humidity of 70% to 100%.

The magnetic recording medium may be, for example, a tape-shapedmagnetic recording medium (magnetic tape). The magnetic tape is normallyused to be accommodated and circulated in a magnetic tape cartridge. Themagnetic tape cartridge is mounted on the magnetic recording andreproducing device, the magnetic tape runs in the magnetic recording andreproducing device to bring the surface of the magnetic tape (surface ofthe magnetic layer) and the magnetic head into contact to slide on eachother, and accordingly, the recording of data on the magnetic tape andreproducing thereof can be performed. However, the magnetic recordingmedium according to one aspect of the invention is not limited to themagnetic tape. The magnetic recording medium according to one aspect ofthe invention is suitable as various magnetic recording media (magnetictape, or disk-shaped magnetic recording medium (magnetic disk) used inthe sliding type magnetic recording and reproducing device. The slidingtype device is a device in which the surface of the magnetic layer andthe head are in contact with each other and slide, in a case ofperforming recording of data on the magnetic recording medium and/orreproducing of the recorded data.

A servo pattern can be formed on the magnetic recording mediummanufactured as described above by a well-known method, in order torealize tracking control of a magnetic head of the magnetic recordingand reproducing device and control of a running speed of the magneticrecording medium. The “formation of the servo pattern” can be “recordingof a servo signal”. The magnetic recording medium may be a tape-shapedmagnetic recording medium (magnetic tape) or a disk-shaped magneticrecording medium (magnetic disk). Hereinafter, the formation of theservo pattern will be described using a magnetic tape as an example.

The servo pattern is generally formed along a longitudinal direction ofthe magnetic tape. As a method of control using a servo signal (servocontrol), timing-based servo (TBS), amplitude servo, or frequency servois used.

As shown in European Computer Manufacturers Association (ECMA)-319, atiming-based servo system is used in a magnetic tape based on alinear-tape-open (LTO) standard (generally referred to as an “LTOtape”). In this timing-based servo system, the servo pattern isconfigured by continuously disposing a plurality of pairs of magneticstripes (also referred to as “servo stripes”) not parallel to each otherin a longitudinal direction of the magnetic tape. As described above, areason for that the servo pattern is configured with one pair ofmagnetic stripes not parallel to each other is because a servo signalreading element passing on the servo pattern recognizes a passageposition thereof. Specifically, one pair of the magnetic stripes areformed so that a gap thereof is continuously changed along the widthdirection of the magnetic tape, and a relative position of the servopattern and the servo signal reading element can be recognized, by thereading of the gap thereof by the servo signal reading element. Theinformation of this relative position can realize the tracking of a datatrack. Accordingly, a plurality of servo tracks are generally set on theservo pattern along the width direction of the magnetic tape.

The servo band is configured of a servo signal continuous in thelongitudinal direction of the magnetic tape. A plurality of servo bandsare normally provided on the magnetic tape. For example, the numberthereof is 5 in the LTO tape. A region interposed between two adjacentservo bands is called a data band. The data band is configured of aplurality of data tracks and each data track corresponds to each servotrack.

In one aspect, as shown in JP2004-318983A, information showing thenumber of servo band (also referred to as “servo band identification(ID)” or “Unique Data Band Identification Method (UDIM) information”) isembedded in each servo band. This servo band ID is recorded by shiftinga specific servo stripe among the plurality of pair of servo stripes inthe servo band so that the position thereof is relatively deviated inthe longitudinal direction of the magnetic tape. Specifically, theposition of the shifted specific servo stripe among the plurality ofpair of servo stripes is changed for each servo band. Accordingly, therecorded servo band ID becomes unique for each servo band, andtherefore, the servo band can be uniquely specified by only reading oneservo band by the servo signal reading element.

In a method of uniquely specifying the servo band, a staggered method asshown in ECMA-319 is used. In this staggered method, the group of onepair of magnetic stripes (servo stripe) not parallel to each other whichare continuously disposed in the longitudinal direction of the magnetictape is recorded so as to be shifted in the longitudinal direction ofthe magnetic tape for each servo band. A combination of this shiftedservo band between the adjacent servo bands is set to be unique in theentire magnetic tape, and accordingly, the servo band can also beuniquely specified by reading of the servo pattern by two servo signalreading elements.

In addition, as shown in ECMA-319, information showing the position inthe longitudinal direction of the magnetic tape (also referred to as“Longitudinal Position (LPOS) information”) is normally embedded in eachservo band. This LPOS information is recorded so that the position ofone pair of servo stripes are shifted in the longitudinal direction ofthe magnetic tape, in the same manner as the UDIM information. However,unlike the UDIM information, the same signal is recorded on each servoband in this LPOS information.

Other information different from the UDIM information and the LPOSinformation can be embedded in the servo band. In this case, theembedded information may be different for each servo band as the UDIMinformation, or may be common in all of the servo bands, as the LPOSinformation.

In addition, as a method of embedding the information in the servo band,a method other than the method described above can be used. For example,a predetermined code may be recorded by thinning out a predeterminedpair among the group of pairs of the servo stripes.

A servo pattern forming head is also referred to as a servo write head.The servo write head includes pairs of gaps corresponding to the pairsof magnetic stripes by the number of servo bands. In general, a core anda coil are respectively connected to each of the pairs of gaps, and amagnetic field generated in the core can generate leakage magnetic fieldin the pairs of gaps, by supplying a current pulse to the coil. In acase of forming the servo pattern, by inputting a current pulse whilecausing the magnetic tape to run on the servo write head, the magneticpattern corresponding to the pair of gaps is transferred to the magnetictape, and the servo pattern can be formed. A width of each gap can besuitably set in accordance with a density of the servo patterns to beformed. The width of each gap can be set as, for example, equal to orsmaller than 1 μm, 1 to 10 μm, or equal to or greater than 10 μm.

Before forming the servo pattern on the magnetic tape, a demagnetization(erasing) process is generally performed on the magnetic tape. Thiserasing process can be performed by applying a uniform magnetic field tothe magnetic tape by using a DC magnet and an AC magnet. The erasingprocess includes direct current (DC) erasing and alternating current(AC) erasing. The AC erasing is performed by slowing decreasing anintensity of the magnetic field, while reversing a direction of themagnetic field applied to the magnetic tape. Meanwhile, the DC erasingis performed by adding the magnetic field in one direction to themagnetic tape. The DC erasing further includes two methods. A firstmethod is horizontal DC erasing of applying the magnetic field in onedirection along a longitudinal direction of the magnetic tape. A secondmethod is vertical DC erasing of applying the magnetic field in onedirection along a thickness direction of the magnetic tape. The erasingprocess may be performed with respect to all of the magnetic tape or maybe performed for each servo band of the magnetic tape.

A direction of the magnetic field to the servo pattern to be formed isdetermined in accordance with the direction of erasing. For example, ina case where the vertical DC erasing is performed to the magnetic tape,the formation of the servo pattern is performed so that the direction ofthe magnetic field and the direction of erasing becomes opposite to eachother. Accordingly, the output of the servo signal obtained by thereading of the servo pattern can be increased. As disclosed inJP2012-053940A, in a case where the magnetic pattern is transferred tothe magnetic tape subjected to the vertical DC erasing by using the gap,the servo signal obtained by the reading of the formed servo pattern hasa unipolar pulse shape. Meanwhile, in a case where the magnetic patternis transferred to the magnetic tape subjected to the horizontal DCerasing by using the gap, the servo signal obtained by the reading ofthe formed servo pattern has a bipolar pulse shape.

Magnetic Recording and Reproducing Device

One aspect of the invention relates to a magnetic recording andreproducing device including the magnetic recording medium and amagnetic head.

In the invention and the specification, the “magnetic recording andreproducing device” means a device capable of performing at least one ofthe recording of data on the magnetic recording medium or thereproducing of data recorded on the magnetic recording medium. Such adevice is generally called a drive. The magnetic recording andreproducing device can be a sliding type magnetic recording andreproducing device. The magnetic head included in the magnetic recordingand reproducing device can be a recording head capable of performing therecording of data on the magnetic recording medium, and can also be areproducing head capable of performing the reproducing of data recordedon the magnetic recording medium. In addition, in the aspect, themagnetic recording and reproducing device can include both of arecording head and a reproducing head as separate magnetic heads. Inanother aspect, the magnetic head included in the magnetic recording andreproducing device can also have a configuration of including both of arecording element and a reproducing element in one magnetic head. As thereproducing head, a magnetic head (MR head) including a magnetoresistive(MR) element capable of reading data recorded on the magnetic recordingmedium with excellent sensitivity as the reproducing element ispreferable. As the MR head, various well-known MR heads can be used. Inaddition, the magnetic head which performs the recording of data and/orthe reproducing of data may include a servo pattern reading element.Alternatively, as a head other than the magnetic head which performs therecording of data and/or the reproducing of data, a magnetic head (servohead) including a servo pattern reading element may be included in themagnetic recording and reproducing device.

In the magnetic recording and reproducing device, the recording of dataon the magnetic recording medium and the reproducing of data recorded onthe magnetic recording medium can be performed by bringing the surfaceof the magnetic layer of the magnetic recording medium into contact withthe magnetic head and sliding. The magnetic recording and reproducingdevice may include the magnetic recording medium according to the aspectof the invention, and well-known technologies can be applied for theother configurations.

EXAMPLES

Hereinafter, the invention will be described with reference to examples.However, the invention is not limited to aspects shown in the examples.“Parts” and “%” in the following description mean “parts by mass” and “%by mass”, unless otherwise noted. In addition, steps and evaluationsdescribed below are performed in an environment of an atmospheretemperature of 23° C.±1° C., unless otherwise noted.

Example 1

A list of each layer forming composition is shown below.

List of Magnetic Layer Forming Composition

-   -   Magnetic Liquid    -   Ferromagnetic powder (see Table 1): 100.0 parts    -   Oleic acid: 2.0 parts    -   Vinyl chloride copolymer (MR-104 manufactured by Kaneka): 10.0        parts    -   SO₃Na group-containing polyurethane resin: 4.0 parts        -   (weight average molecular weight: 70,000, SO₃Na group: 0.07            meq/g)    -   Polyalkylenimine-based polymer (synthetic product obtained by        the method disclosed in paragraphs 0115 to 0123 of        JP2016-051493A): 6.0 parts    -   Methyl ethyl ketone: 150.0 parts    -   Cyclohexanone: 150.0 parts    -   Abrasive Liquid    -   α-alumina (BET (Brunauer-Emmett-Teller) specific surface area:        19 m²/g): 6.0 parts    -   SO₃Na group-containing polyurethane resin: 0.6 parts        -   (Weight average molecular weight: 70,000, SO₃Na group: 0.1            meq/g)    -   2,3-dihydroxynaphthalene: 0.6 parts    -   Cyclohexanone: 23.0 parts    -   Projection Formation Agent Liquid    -   Colloidal silica (average particle size: 120 nm): 2.0 parts    -   Methyl ethyl ketone: 8.0 parts    -   Other components    -   Stearic acid: 3.0 parts    -   Stearic acid amide: 0.3 parts    -   Butyl stearate: 6.0 parts    -   Methyl ethyl ketone: 110.0 parts    -   Cyclohexanone: 110.0 parts    -   Polyisocyanate (CORONATE (registered trademark) L manufactured        by Tosoh Corporation): 3 parts

List of Non-Magnetic Layer Forming Composition

-   -   Non-magnetic inorganic powder    -   α-iron oxide (average particle size: 10 nm, BET specific surface        area: 75 m²/g): 100.0 parts    -   Carbon black (average particle size: 20 nm): 25.0 parts    -   SO₃Na group-containing polyurethane resin (weight-average        molecular weight: 70,000, SO₃Na group: 0.2 meq/g): 18.0 parts    -   Stearic acid: 1.0 part    -   Cyclohexanone: 300.0 parts    -   Methyl ethyl ketone: 300.0 parts

List of Back Coating Layer Forming Composition

-   -   Non-magnetic powder: 100.0 parts    -   α-iron oxide: see Table 1 as mixing ratio (mass ratio)        -   average particle size (average long axis length): 150 nm    -   average acicular ratio: 7    -   BET specific surface area: 52 m2/g    -   Carbon black: see Table 1 as mixing ratio (mass ratio)    -   Average particle size: 20 nm    -   A vinyl chloride copolymer (MR-104 manufactured by Kaneka        Corporation): 13.0 parts        -   (Weight-average molecular weight: 55,000, active            hydrogen-containing group (hydroxy group): 0.33 meq/g, OSO3K            group (potassium salt of sulfate group): 0.09 meq/g)    -   SO3Na group-containing polyurethane resin: 6.0 parts        -   (Weight-average molecular weight: 70,000, active            hydrogen-containing group (hydroxy group): 4 to 6 mgKOH/g,            SO₃Na group (sodium salt of sulfonic acid group): 0.07            meq/g)    -   Phenylphosphonic acid: 3.0 parts    -   Cyclohexanone: 155.0 parts    -   Methyl ethyl ketone: 155.0 parts    -   Stearic acid: 3.0 parts    -   Butyl stearate: 3.0 parts    -   Polyisocyanate: 5.0 parts    -   Cyclohexanone: 200.0 parts

Preparation of Magnetic Layer Forming Composition

The magnetic layer forming composition was prepared by the followingmethod.

A magnetic liquid was prepared by dispersing (beads-dispersing) variouscomponents of the magnetic liquid with a batch type vertical sand millfor 24 hours. As dispersion beads, zirconia beads having a bead diameterof 0.5 mm were used.

Regarding the abrasive solution, various components of the abrasivesolution were mixed with each other and put in a transverse beads milldisperser together with zirconia beads having a bead diameter of 0.3 mm,so as to perform the adjustment so that a value of bead volume/(abrasivesolution volume+bead volume) was 80%, the beads mill dispersion processwas performed for 120 minutes, the liquid after the process wasextracted, and an ultrasonic dispersion filtering process was performedby using a flow type ultrasonic dispersion filtering device. By doingso, the abrasive solution was prepared.

The prepared magnetic liquid, the abrasive solution, the projectionformation agent liquid, and the other components were introduced in adissolver stirrer, and stirred at a circumferential speed of 10 m/secfor 30 minutes. Then, a process at a flow rate of 7.5 kg/min wasperformed for 3 passes with a flow type ultrasonic disperser, and then,the mixture was filtered with a filter having a hole diameter of 1 μm,to prepare a magnetic layer forming composition.

Preparation of Non-Magnetic Layer Forming Composition

A non-magnetic layer forming composition was prepared by dispersingvarious components of the non-magnetic layer forming compositiondescribed above with a batch type vertical sand mill by using zirconiabeads having a bead diameter of 0.1 mm for 24 hours, and then performingfiltering with a filter having an average hole diameter of 0.5 μm.

Preparation of Back Coating Layer Forming Composition

Components except a lubricant (stearic acid and butyl stearate),polyisocyanate, and 200.0 parts of cyclohexanone among variouscomponents of the back coating layer forming composition were kneadedand diluted by an open kneader, and subjected to a dispersion process of12 passes, with a transverse beads mill disperser and zirconia beadshaving a bead diameter of 1 mm, by setting a bead filling percentage as80 volume %, a circumferential speed of rotor distal end as 10 m/sec,and a retention time for 1 pass as 2 minutes. After that, the remainingcomponents were added and stirred with a dissolver, the obtaineddispersion liquid was filtered with a filter having an average holediameter of 1 μm and a back coating layer forming composition wasprepared.

Manufacturing of Magnetic Tape

The non-magnetic layer forming composition prepared as described abovewas applied onto a surface of an aromatic polyamide support having athickness of 8.0 μm shown in Table 1, so that the thickness after thedrying becomes 400 nm, and dried to form a non-magnetic layer. Then, themagnetic layer forming composition prepared as described above wasapplied onto a surface of the non-magnetic layer so that the thicknessafter the drying becomes 70 nm, to form a coating layer. A homeotropicalignment process of applying a magnetic field having strength of 0.3 Tto the surface of the coating layer in a vertical direction while thecoating layer of the magnetic layer forming composition is wet (notdried), and the coating layer was dried. After that, the back coatinglayer forming composition prepared as described above was applied on theopposite surface of the support so that the thickness after dryingbecomes 0.4 μm, and dried. By doing so, a magnetic tape original rollwas manufactured.

The calender process (surface smoothing treatment) was performed on themanufactured magnetic tape original roll with a calender configured ofonly a metal roll, at a speed of 100 m/min, linear pressure of 294 kN/m,and a surface temperature of a calender roll of 100° C., and heatingprocess was performed in the environment of the atmosphere temperatureshown in Table 1 for a period of time shown in Table 1. After theheating process, a magnetic tape having a width of ½ inches was obtainedby slitting the magnetic tape original roll with a cutter. While causingthis magnetic tape to run between a sending roller and a winding roller(running speed: 120 m/min, tension: see Table 1), blade polishing of thesurface of the back coating layer, the dry wiping treatment, and themethyl ethyl ketone wiping treatment were performed in this order.Specifically, a sapphire blade, a dried wiping material (TORAYSEE(registered trademark) manufactured by Toray Industries, Inc.), and awiping material permeated with methyl ethyl ketone (TORAYSEE (registeredtrademark) manufactured by Toray Industries, Inc.) were disposed betweenthe two rollers described above, the sapphire blade was pressed againstthe surface of the back coating layer of the magnetic tape runningbetween the two rollers for blade polishing, the dry wiping treatment ofthe surface of the back coating layer was performed with the driedwiping material, and the methyl ethyl ketone wiping treatment of thesurface of the back coating layer was performed with the wiping materialpermeated with methyl ethyl ketone. By doing so, the blade polishing,the dry wiping treatment, and the methyl ethyl ketone wiping treatmentwere performed on the surface of the back coating layer once.

In a state where the magnetic layer of the manufactured magnetic tapewas demagnetized, servo patterns having disposition and shapes accordingto the linear-tape-open (LTO) Ultrium format were formed on the magneticlayer by using a servo write head mounted on a servo writer.

Accordingly, a magnetic tape including data bands, servo bands, andguide bands in the disposition according to the LTO Ultrium format inthe magnetic layer, and including servo patterns having the dispositionand the shape according to the LTO Ultrium format on the servo band wasobtained.

By doing so, a magnetic tape of Example 1 was obtained.

Examples 2 to 11 and Comparative Examples 1 to 7

Magnetic tapes were manufactured by the same method as in Example 1,except that various conditions were changed as shown in Table 1.

Regarding the surface treatment of the surface of the back coating layerafter slitting, the following surface treatments were performed inExamples 2 to 11 and Comparative Examples 1 to 7, respectively.

In Examples 2 to 4 and 7 to 11 and Comparative Examples 6 and 7, theblade polishing, the dry wiping treatment, and the methyl ethyl ketonewiping treatment were performed in the same manner as in Example 1.

In Examples 5 and 6 and Comparative Example 4, the blade polishing, thedry wiping treatment, and the methyl ethyl ketone wiping treatment wereperformed in the same manner as in Example 1, except that the tensionwas changed.

In Comparative Examples 1, 3 and 5, the blade polishing and the drywiping treatment were repeatedly performed in the same manner as inExample 1, and the methyl ethyl ketone wiping treatment was notperformed.

In Comparative Example 2, the blade polishing and the dry wipingtreatment were performed in the same manner as in Example 1 three times,and methyl ethyl ketone wiping treatment was not performed.

In Table 1, “BaFe” is a hexagonal barium ferrite powder having anaverage particle size (average plate diameter) of 21 nm.

In Table 1, “SrFe1” is a hexagonal strontium ferrite powder produced bythe following method.

1707 g of SrCO₃, 687 g of H3BO3, 1120 g of Fe2O3, 45 g of Al(OH)3, 24 gof BaCO3, 13 g of CaCO3, and 235 g of Nd2O3 were weighed and mixed in amixer to obtain a raw material mixture.

The obtained raw material mixture was dissolved in a platinum crucibleat a dissolving temperature of 1390° C., and a tap hole provided on thebottom of the platinum crucible was heated while stirring the dissolvedliquid, and the dissolved liquid was tapped in a rod shape atapproximately 6 g/sec. The tap liquid was rolled and cooled with a watercooling twin roller to prepare an amorphous body.

280 g of the prepared amorphous body was put into an electronic furnace,heated to 635° C. (crystallization temperature) at a rate of temperaturerise of 3.5° C./min, and held at the same temperature for 5 hours, andhexagonal strontium ferrite particles were precipitated (crystallized).

Then, the crystallized material obtained as described above includingthe hexagonal strontium ferrite particles was coarse-pulverized with amortar, 1000 g of zirconia beads having a particle diameter of 1 mm and800 ml of an acetic acid aqueous solution having a concentration of 1%were added to a glass bottle, and a dispersion process was performed ina paint shaker for 3 hours. After that, the obtained dispersion liquidand the beads were dispersed and put in a stainless still beaker. Thedispersion liquid was left at a liquid temperature of 100° C. for 3hours, subjected to a dissolving process of a glass component,precipitated with a centrifugal separator, decantation was repeated forcleaning, and drying was performed in a heating furnace at a furnaceinner temperature of 110° C. for 6 hours, to obtain hexagonal strontiumferrite powder.

Regarding the hexagonal strontium ferrite powder obtained as describedabove, an average particle size was 18 nm, an activation volume was 902nm³, an anisotropy constant Ku was 2.2×105 J/m³, and a massmagnetization σs was 49 A·m²/kg.

12 mg of a sample powder was collected from the hexagonal strontiumferrite powder obtained as described above, the element analysis of afiltrate obtained by the partial dissolving of this sample powder underthe dissolving conditions described above was performed by the ICPanalysis device, and a surface layer portion content of a neodymium atomwas obtained.

Separately, 12 mg of a sample powder was collected from the hexagonalstrontium ferrite powder obtained as described above, the elementanalysis of a filtrate obtained by the total dissolving of this samplepowder under the dissolving conditions described above was performed bythe ICP analysis device, and a bulk content of a neodymium atom wasobtained.

The content (bulk content) of the neodymium atom in the hexagonalstrontium ferrite powder obtained as described above with respect to 100atom % of iron atom was 2.9 atom %. In addition, the surface portioncontent of the neodymium atom was 8.0 atom %. A ratio of the surfaceportion content and the bulk content, “surface portion content/bulkcontent” was 2.8 and it was confirmed that the neodymium atom isunevenly distributed on the surface layer of the particles.

A crystal structure of the hexagonal ferrite shown by the powderobtained as described above was confirmed by scanning CuKα ray under thecondition of a voltage of 45 kV and intensity of 40 mA and measuring anX-ray diffraction pattern under the following conditions (X-raydiffraction analysis). The powder obtained as described above showed acrystal structure of magnetoplumbite type (M type) hexagonal ferrite. Inaddition, a crystal phase detected by the X-ray diffraction analysis wasa magnetoplumbite type single phase.

PANalytical X'Pert Pro Diffractometer, PIXcel Detector

Soller slit of incident beam and diffraction beam: 0.017 radians

Fixed angle of dispersion slit: ¼ degrees

Mask: 10 mm

Scattering prevention slit: ¼ degrees

Measurement mode: continuous

Measurement time per 1 stage: 3 seconds

Measurement speed: 0.017 degrees per second

Measurement step: 0.05 degrees

In Table 1, “SrFe2” is a hexagonal strontium ferrite powder produced bythe following method.

1725 g of SrCO₃, 666 g of H3BO3, 1332 g of Fe2O3, 52 g of Al(OH)3, 34 gof CaCO3, and 141 g of BaCO3 were weighed and mixed in a mixer to obtaina raw material mixture.

The obtained raw material mixture was dissolved in a platinum crucibleat a dissolving temperature of 1380° C., and a tap hole provided on thebottom of the platinum crucible was heated while stirring the dissolvedliquid, and the dissolved liquid was tapped in a rod shape atapproximately 6 g/sec. The tap liquid was rolled and cooled with a watercooling twin roll to prepare an amorphous body.

280 g of the obtained amorphous body was put into an electronic furnace,heated to 645° C. (crystallization temperature), and held at the sametemperature for 5 hours, and hexagonal strontium ferrite particles wereprecipitated (crystallized).

Then, the crystallized material obtained as described above includingthe hexagonal strontium ferrite particles was coarse-pulverized with amortar, 1000 g of zirconia beads having a particle diameter of 1 mm and800 ml of an acetic acid aqueous solution having a concentration of 1%were added to a glass bottle, and a dispersion process was performed ina paint shaker for 3 hours. After that, the obtained dispersion liquidand the beads were dispersed and put in a stainless still beaker. Thedispersion liquid was left at a liquid temperature of 100° C. for 3hours, subjected to a dissolving process of a glass component,precipitated with a centrifugal separator, decantation was repeated forcleaning, and drying was performed in a heating furnace at a furnaceinner temperature of 110° C. for 6 hours, to obtain hexagonal strontiumferrite powder.

Regarding the hexagonal strontium ferrite powder obtained as describedabove, an average particle size was 19 nm, an activation volume was 1102nm³, an anisotropy constant Ku was 2.0×105 J/m³, and a massmagnetization σs was 50 A·m²/kg.

In Table 1, “ε-iron oxide” is an ε-iron oxide powder produced by thefollowing method.

4.0 g of ammonia aqueous solution having a concentration of 25% wasadded to a material obtained by dissolving 8.3 g of iron (III) nitratenonahydrate, 1.3 g of gallium (III) nitrate octahydrate, 190 mg ofcobalt (II) nitrate hexahydrate, 150 mg of titanium (IV) sulfate, and1.5 g of polyvinyl pyrrolidone (PVP) in 90 g of pure water, whilestirring by using a magnetic stirrer, in an atmosphere under theconditions of an atmosphere temperature of 25° C., and the mixture wasstirred for 2 hours still under the temperature condition of theatmosphere temperature of 25° C. A citric acid solution obtained bydissolving 1 g of citric acid in 9 g of pure water was added to theobtained solution and stirred for 1 hour. The powder precipitated afterthe stirring was collected by centrifugal separation, washed with purewater, and dried in a heating furnace at a furnace inner temperature of80° C.

800 g of pure water was added to the dried powder and the powder wasdispersed in water again, to obtain a dispersion liquid. The obtaineddispersion liquid was heated to a liquid temperature of 50° C., and 40 gof ammonia aqueous solution having a concentration of 25% was addeddropwise while stirring. The stirring was performed for 1 hour whileholding the temperature of 50° C., and 14 mL of tetraethoxysilane (TEOS)was added dropwise and stirred for 24 hours. 50 g of ammonium sulfatewas added to the obtained reaction solution, the precipitated powder wascollected by centrifugal separation, washed with pure water, and driedin a heating furnace at a furnace inner temperature of 80° C. for 24hours, and a precursor of ferromagnetic powder was obtained.

The heating furnace at a furnace inner temperature of 1000° C. wasfilled with the obtained precursor of ferromagnetic powder in theatmosphere and subjected to thermal treatment for 4 hours.

The thermal-treated precursor of ferromagnetic powder was put intosodium hydroxide (NaOH) aqueous solution having a concentration of 4mol/L, the liquid temperature was held at 70° C., stirring was performedfor 24 hours, and accordingly, a silicon acid compound which was animpurity was removed from the thermal-treated precursor of ferromagneticpowder.

After that, by the centrifugal separation process, ferromagnetic powderobtained by removing the silicon acid compound was collected and washedwith pure water, and ferromagnetic powder was obtained.

The composition of the obtained ferromagnetic powder was confirmed byInductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES), andGa, Co, and Ti substitution type ε-iron oxide(ε-Ga_(0.28)Co_(0.05)Ti_(0.05)Fe_(1.62)O₃) was obtained. In addition,the X-ray diffraction analysis was performed under the same conditionsas disclosed regarding SrFe1 above, and it was confirmed that theobtained ferromagnetic powder has a crystal structure of a single phasewhich is an c phase not including a crystal structure of an α phase anda γ phase (ε-iron oxide type crystal structure) from the peak of theX-ray diffraction pattern.

Regarding the obtained (α-iron oxide powder, an average particle sizewas 12 nm, an activation volume was 746 nm³, an anisotropy constant Kuwas 1.2×105 J/m³, and a mass magnetization σs was 16 A·m²/kg.

The activation volume and the anisotropy constant Ku of the hexagonalstrontium ferrite powder and the (ε-iron oxide powder are valuesobtained by the method described above regarding each magnetic powder byusing an oscillation sample type magnetic-flux meter (manufactured byToei Industry Co., Ltd.).

The mass magnetization σs is a value measured using a oscillation sampletype magnetic-flux meter (manufactured by Toei Industry Co., Ltd.) at amagnetic field strength of 15 kOe.

In Table 1, “Support 1” shown in the column of type of aromaticpolyamide support is an aromatic polyamide film produced by the methoddisclosed in Example 7 of JP1997-176306A (JP-H9-176306A).

“Support 2” is an aromatic polyamide film produced by the methoddisclosed in Example 6 of JP1997-176306A (JP-H9-176306A).

“Support 3” is an aromatic polyamide film produced by the methoddisclosed in Example 3 of JP1997-176306A (JP-H9-176306A).

“Support 4” is an aromatic polyamide film produced by the methoddisclosed in Comparative Example 3 of JP1997-176306A (JP-H9-176306A).

A test piece of several grams was cut out from each aromatic polyamidesupport, and the moisture absorption was obtained by the methoddescribed above and was a value shown in Table 1.

Evaluation Method

(1) Spacing Difference (S_(after)−S_(before)) Before and after MethylEthyl Ketone Cleaning

The spacing difference (S_(after)−S_(before)) before and after themethyl ethyl ketone cleaning was obtained with a Tape Spacing Analyzer(TSA) (manufactured by Micro Physics, Inc.) by the following method.

Two test pieces having a length of 5 cm were cut out from each magnetictape of the examples and the comparative examples. Regarding one testpiece, the methyl ethyl ketone cleaning was not performed and thespacing (S_(before)) was obtained by the following method. Regarding theother test piece, the methyl ethyl ketone cleaning was performed by themethod described above, and the spacing (S_(after)) was obtained by thefollowing method.

In a state where a glass plate (glass plate (model no.: WG10530)manufactured by Thorlabs, Inc.) included in TSA is disposed on thesurface of the back coating layer of the magnetic tape (specifically,the test piece), a urethane hemisphere included in TSA as a pressingmember was pressed against the surface of the magnetic layer of themagnetic tape with pressure of 0.5 atm. In this state, a certain region(150,000 to 200,000 μm²) of the surface of the back coating layer of themagnetic tape was irradiated with white light from a stroboscopeincluded in the TSA through the glass plate, the obtained reflectedlight was received with a charge-coupled device (CCD) through aninterference filter (filter selectively transmitting light at awavelength of 633 nm), thereby obtaining an interference fringe imagegenerated on ruggedness of this region.

This image was divided into 300,000 points, a distance (spacing) betweenthe surface of the glass plate of each point on the magnetic tape sideand the surface of the back coating layer of the magnetic tape wasobtained, this spacing is shown with a histogram, a mode S_(before) ofthe histogram obtained regarding the test piece not subjected to themethyl ethyl ketone cleaning was subtracted from a mode S_(after) of thehistogram obtained regarding the test piece after the methyl ethylketone cleaning, and the difference (S_(after)−S_(before)) was obtained.

(2) Spacing Difference (S_(reference)−S_(before)) Before and afterMethyl n-Hexane (Reference Value)

One test piece having a length of 5 cm was further cut out from eachmagnetic tape of the examples and the comparative examples, the cleaningwas performed in the same manner as described above, except thatn-hexane was used instead of methyl ethyl ketone, and the spacing wasobtained after n-hexane cleaning in the same manner as described above.A difference (S_(reference)−S_(before)) between the spacingS_(reference) obtained here, and the spacing S_(before) obtained fromthe test piece not subjected to the cleaning obtained in the section of(1) was obtained as a reference value.

(3) Evaluation of Deterioration in Running Stability

Each magnetic tape of the examples and the comparative examples wasstored in a thermo box in which a temperature was 10° C. and relativehumidity was 80%, for 3 hours. After that, the magnetic tape wasextracted from the thermo box (in the outside air, a temperature was 23°C. and relative humidity was 50%), and put in a thermo room in which atemperature was 32° C. and relative humidity was 80% within 1 minute,and a position error signal (PES) was obtained by the following methodin the thermo room for a week.

Regarding each magnetic tape of the examples and the comparativeexamples, a servo pattern was read with a verifying head on a servowriter used in the formation of the servo pattern. The verifying head isa magnetic head for reading for confirming quality of the servo patternformed in the magnetic tape, and an element for reading is disposed on aposition corresponding to the position (position of the magnetic tape ina width direction) of the servo pattern, in the same manner as themagnetic head of the well-known magnetic tape device (drive).

In the verifying head, a well-known PES arithmetic circuit whichcalculates head positioning accuracy of the servo system as the PES isconnected from an electrical signal obtained by reading the servopattern in the verifying head. The PES arithmetic circuit calculates, ina case where necessary, displacement of the magnetic tape in a widthdirection from the input electrical signal (pulse signal), and a valueobtained by applying a high pass filter (cut-off: 500 cycles/m) withrespect to a temporal change information (signal) of this displacementwas calculated as the PES. The PES can be an index for runningstability, and in a case where the calculated PES is equal to or smallerthan 18 nm, it is possible to evaluate that a deterioration in runningstability due to storage in a high temperature and a high humidityenvironment after a temperature change from a low temperature to a hightemperature under high humidity is prevented.

The result described above is shown in Table 1 (Tables 1-1 to 1-3).

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Ferromagneticpowder BaFe BaFe BaFe BaFe BaFe Mixing ratio of non-magnetic 60/40 80/20100/0 60/40 60/40 powder in back coating layer (α-iron oxide/carbonblack) Heating Temperature 70° C. 70° C. 70° C. 80° C. 80° C. processTime 36 hours 36 hours 36 hours 48 hours 48 hours Tension (N) 0.2940.294 0.294 0.294 0.588 Blade polishing and dry wiping 1 time 1 time 1time 1 time 1 time treatment Methyl ethyl ketone wiping PerformedPerformed Performed Performed Performed treatment (Reference value) 4.04.0 4.0 4.0 4.0 Spacing difference (S_(reference) − S_(before)) beforeand after n-hexane cleaning (nm) Spacing difference (S_(after) − 28.025.0 26.0 21.0 11.0 S_(before)) before and after methyl ethyl ketonecleaning (nm) Aromatic Type Support 1 Support 1 Support 1 Support 1Support 1 polyamide Hygroscopic 1.4% 1.4% 1.4% 1.4% 1.4% supportcoefficient Evaluation of deterioration in 15 nm 12 nm 14 nm 10 nm 7 nmrunning stability (PES) Example 6 Example 7 Example 8 Example 9 Example10 Example 11 Ferromagnetic powder BaFe BaFe BaFe SrFe1 SrFe2 Mixingratio of non-magnetic 60/40 80/20 80/20 60/40 60/40 60/40 powder in backcoating layer (α-iron oxide/carbon black) Heating Temperature 80° C. 70°C. 70° C. 70° C. 70° C. 70° C. process Time 48 hours 36 hours 36 hours36 hours 36 hours 36 hours Tension (N) 1.176 0.294 0.294 0.294 0.2940.294 Blade polishing and dry wiping 1 time 1 time 1 time 1 time 1 time1 time treatment Methyl ethyl ketone wiping Performed PerformedPerformed Performed Performed Performed treatment (Reference value) 2.04.0 4.0 4.0 4.0 4.0 Spacing difference (S_(reference) − S_(before))before and after n-hexane cleaning (nm) Spacing difference (S_(a)f_(ter)− 4.0 25.0 25.0 28.0 28.0 28.0 S_(before)) before and after methyl ethylketone cleaning (nm) Aromatic Type Support 1 Support 2 Support 3 Support1 Support 1 Support 1 polyamide Hygroscopic 1.4% 1.9% 1.7% 1.4% 1.4%1.4% support coefficient Evaluation of deterioration in 5 nm 15 nm 15 nm15 nm 15 nm 15 nm running stability (PES) Comparative ComparativeComparative Comparative Comparative Comparative Comparative Example 1Example 2 Example 3 Example 4 Example 5 Example 6 Example 7Ferromagnetic powder BaFe BaFe BaFe BaFe BaFe BaFe BaFe Mixing ratio ofnon-magnetic 60/40 60/40 60/40 60/40 60/40 60/40 80/20 powder in backcoating layer (α-iron oxide/carbon black) Heating Temperature 60° C. 60°C. 70° C. 80° C. 70° C. 70° C. 70° C. process Time 24 hours 24 hours 36hours 48 hours 36 hours 36 hours 36 hours Tension (N) 0.294 0.294 0.2941.960 0.294 0.294 0.294 Blade polishing and dry 1 time 3 times 1 time 1time 1 time 1 time 1 time wiping treatment Methyl ethyl ketone wipingNot Not Not Performed Not Performed Performed treatment performedperformed performed performed (Reference value) 4.0 4.0 4.0 0 4.0 4.04.0 Spacing difference (S_(reference)− S_(before)) before and aftern-hexane cleaning (nm) Spacing difference (S_(after) − 34.0 33.0 32.0 032.0 28.0 25.0 S_(before)) before and after methyl ethyl ketone cleaning(nm) Aromatic Type Support 4 Support 4 Support 4 Support 4 Support 1Support 4 Support 4 polyamide Hygroscopic 2.3% 2.3% 2.3% 2.3% 1.4% 2.3%2.3% support coefficient Evaluation of deterioration 40 nm 35 nm 35 nm50 nm 25 nm 25 nm 22 nm in running stability (PES)

The magnetic tape of the examples is a magnetic tape having an aromaticpolyamide support. From the values of PES in Table 1, in the magnetictape of the example, it can be confirmed that a deterioration in runningstability after being stored in a high temperature and high humidityenvironment after a temperature change from a low temperature to a hightemperature under high humidity is prevented.

In addition, as shown in Table 1, there was no correlation between thevalue of the spacing difference (S_(reference)−S_(before)) before andafter n-hexane cleaning and the value of the spacing difference(S_(after)−S_(before)) before and after methyl ethyl ketone cleaning.

One aspect of the invention is effective in a technical field of amagnetic recording medium for various data storage.

What is claimed is:
 1. A magnetic recording medium comprising: anon-magnetic support; a magnetic layer including a ferromagnetic powderon one surface of the non-magnetic support; and a back coating layerincluding a non-magnetic powder on the other surface of the non-magneticsupport, wherein a difference (S_(after)−S_(before)) between a spacingS_(after) measured by optical interferometry regarding a surface of theback coating layer after methyl ethyl ketone cleaning and a spacingS_(before) measured by optical interferometry regarding the surface ofthe back coating layer before methyl ethyl ketone cleaning is greaterthan 0 nm and equal to or smaller than 30.0 nm, and the non-magneticsupport is an aromatic polyamide support having a moisture absorption of2.2% or less.
 2. The magnetic recording medium according to claim 1,wherein the difference (S_(after)−S_(before)) is 4.0 nm to 28.0 nm. 3.The magnetic recording medium according to claim 1, wherein the aromaticpolyamide support has a moisture absorption of 2.0% or less.
 4. Themagnetic recording medium according to claim 1, wherein the aromaticpolyamide support has a moisture absorption of 1.0% or more and 2.0% orless.
 5. The magnetic recording medium according to claim 1, furthercomprising: a non-magnetic layer including a non-magnetic powder betweenthe non-magnetic support and the magnetic layer.
 6. The magneticrecording medium according to claim 1, wherein the magnetic recordingmedium is a magnetic tape.
 7. A magnetic recording and reproducingdevice comprising: the magnetic recording medium according to claim 1;and a magnetic head.
 8. The magnetic recording and reproducing deviceaccording to claim 7, wherein the difference (S_(after)−S_(before)) is4.0 nm to 28.0 nm.
 9. The magnetic recording and reproducing deviceaccording to claim 7, wherein the aromatic polyamide support has amoisture absorption of 2.0% or less.
 10. The magnetic recording andreproducing device according to claim 7, wherein the aromatic polyamidesupport has a moisture absorption of 1.0% or more and 2.0% or less. 11.The magnetic recording and reproducing device according to claim 7,wherein the magnetic recording medium further comprises a non-magneticlayer including a non-magnetic powder between the non-magnetic supportand the magnetic layer.
 12. The magnetic recording and reproducingdevice according to claim 7, wherein the magnetic recording medium is amagnetic tape.